In this, it's argued that two-weapon fighting is a terrible choice for a ranger because fighting with paired short swords yields the same max damage as an identical ranger without the two weapon fighting feats holding a greatsword. This has a couple flaws with it. First, pair short swords are not nearly as good as longsword/short sword. Second, it's not max damage, but expected damage that matters.
I did some math on a few builds. I assumed the two-weapon fighting ranger takes two-weapon fighting at first level and double slice at second. At 6th level, he takes greater two-weapon fighting. He's using a longsword and a short sword. The greatsword wielder has no feats at all, as far as we're concerned. Both characters have 18 strength. Here's the expected damage for each against ACs from 10 to 30. The number at the beginning of each set is the mean across all the armor classes.
Two handed, level 1: 3.8955 10 *************************************************** 10.22 11 *********************************************** 9.5 12 ******************************************* 8.78 13 **************************************** 8.06 14 ************************************ 7.34 15 ********************************* 6.62 16 ***************************** 5.9 17 ************************* 5.18 18 ********************** 4.46 19 ****************** 3.74 20 *************** 3.02 21 *********** 2.3 22 ******* 1.58 23 **** 0.86 24 0.14 25 0.035 26 0.035 27 0.035 28 0.035 29 0.035 30 0.035 Two weapon, level 1: 3.162 10 *********************************************** 9.52 11 ******************************************* 8.74 12 *************************************** 7.96 13 *********************************** 7.18 14 ******************************** 6.4 15 **************************** 5.62 16 ************************ 4.84 17 ******************** 4.06 18 **************** 3.28 19 ************ 2.5 20 ******** 1.72 21 **** 0.94 22 0.16 23 0.04 24 0.04 25 0.04 26 0.04 27 0.04 28 0.04 29 0.04 30 0.04 Two handed, level 2: 4.44075 10 ****************************************************** 10.94 11 *************************************************** 10.22 12 *********************************************** 9.5 13 ******************************************* 8.78 14 **************************************** 8.06 15 ************************************ 7.34 16 ********************************* 6.62 17 ***************************** 5.9 18 ************************* 5.18 19 ********************** 4.46 20 ****************** 3.74 21 *************** 3.02 22 *********** 2.3 23 ******* 1.58 24 **** 0.86 25 0.14 26 0.035 27 0.035 28 0.035 29 0.035 30 0.035 Two weapon, level 2: 4.13 10 ********************************************************** 11.6 11 ***************************************************** 10.72 12 ************************************************* 9.84 13 ******************************************** 8.96 14 **************************************** 8.08 15 ************************************ 7.2 16 ******************************* 6.32 17 *************************** 5.44 18 ********************** 4.56 19 ****************** 3.68 20 ************** 2.8 21 ********* 1.92 22 ***** 1.04 23 0.16 24 0.04 25 0.04 26 0.04 27 0.04 28 0.04 29 0.04 30 0.04 Two handed, level 3: 5.022 10 ********************************************************** 11.66 11 ****************************************************** 10.94 12 *************************************************** 10.22 13 *********************************************** 9.5 14 ******************************************* 8.78 15 **************************************** 8.06 16 ************************************ 7.34 17 ********************************* 6.62 18 ***************************** 5.9 19 ************************* 5.18 20 ********************** 4.46 21 ****************** 3.74 22 *************** 3.02 23 *********** 2.3 24 ******* 1.58 25 **** 0.86 26 0.14 27 0.035 28 0.035 29 0.035 30 0.035 Two weapon, level 3: 4.752 10 ************************************************************** 12.48 11 ********************************************************** 11.6 12 ***************************************************** 10.72 13 ************************************************* 9.84 14 ******************************************** 8.96 15 **************************************** 8.08 16 ************************************ 7.2 17 ******************************* 6.32 18 *************************** 5.44 19 ********************** 4.56 20 ****************** 3.68 21 ************** 2.8 22 ********* 1.92 23 ***** 1.04 24 0.16 25 0.04 26 0.04 27 0.04 28 0.04 29 0.04 30 0.04 Two handed, level 4: 5.63925 10 ************************************************************* 12.38 11 ********************************************************** 11.66 12 ****************************************************** 10.94 13 *************************************************** 10.22 14 *********************************************** 9.5 15 ******************************************* 8.78 16 **************************************** 8.06 17 ************************************ 7.34 18 ********************************* 6.62 19 ***************************** 5.9 20 ************************* 5.18 21 ********************** 4.46 22 ****************** 3.74 23 *************** 3.02 24 *********** 2.3 25 ******* 1.58 26 **** 0.86 27 0.14 28 0.035 29 0.035 30 0.035 Two weapon, level 4: 5.418 10 ****************************************************************** 13.36 11 ************************************************************** 12.48 12 ********************************************************** 11.6 13 ***************************************************** 10.72 14 ************************************************* 9.84 15 ******************************************** 8.96 16 **************************************** 8.08 17 ************************************ 7.2 18 ******************************* 6.32 19 *************************** 5.44 20 ********************** 4.56 21 ****************** 3.68 22 ************** 2.8 23 ********* 1.92 24 ***** 1.04 25 0.16 26 0.04 27 0.04 28 0.04 29 0.04 30 0.04 Two handed, level 5: 6.2565 10 ************************************************************* 12.38 11 ************************************************************* 12.38 12 ********************************************************** 11.66 13 ****************************************************** 10.94 14 *************************************************** 10.22 15 *********************************************** 9.5 16 ******************************************* 8.78 17 **************************************** 8.06 18 ************************************ 7.34 19 ********************************* 6.62 20 ***************************** 5.9 21 ************************* 5.18 22 ********************** 4.46 23 ****************** 3.74 24 *************** 3.02 25 *********** 2.3 26 ******* 1.58 27 **** 0.86 28 0.14 29 0.035 30 0.035 Two weapon, level 5: 6.128 10 *********************************************************************** 14.24 11 ****************************************************************** 13.36 12 ************************************************************** 12.48 13 ********************************************************** 11.6 14 ***************************************************** 10.72 15 ************************************************* 9.84 16 ******************************************** 8.96 17 **************************************** 8.08 18 ************************************ 7.2 19 ******************************* 6.32 20 *************************** 5.44 21 ********************** 4.56 22 ****************** 3.68 23 ************** 2.8 24 ********* 1.92 25 ***** 1.04 26 0.16 27 0.04 28 0.04 29 0.04 30 0.04 Two handed, level 6: 9.30925 10 ************************************************************************************************* 19.5 11 ********************************************************************************************** 18.88 12 ******************************************************************************************* 18.26 13 ************************************************************************************* 17.02 14 ****************************************************************************** 15.78 15 ************************************************************************ 14.54 16 ****************************************************************** 13.3 17 ************************************************************ 12.06 18 ****************************************************** 10.82 19 *********************************************** 9.58 20 ***************************************** 8.34 21 *********************************** 7.1 22 ***************************** 5.86 23 *********************** 4.62 24 **************** 3.38 25 ************* 2.655 26 ********** 2.035 27 ******* 1.415 28 *** 0.795 29 0.175 30 0.07 Two weapon, level 6: 10.434 10 ********************************************************************************************************************************* 25.84 11 ************************************************************************************************************************ 24.08 12 *************************************************************************************************************** 22.32 13 ****************************************************************************************************** 20.56 14 ********************************************************************************************** 18.8 15 ************************************************************************************* 17.04 16 **************************************************************************** 15.28 17 ******************************************************************* 13.52 18 ********************************************************** 11.76 19 ************************************************** 10.0 20 ***************************************** 8.24000000001 21 ******************************** 6.48 22 *********************** 4.72 23 ****************** 3.72 24 ************** 2.84 25 ********* 1.96 26 ***** 1.08 27 * 0.2 28 0.08 29 0.08 30 0.08 Two handed, level 7: 10.31175 10 **************************************************************************************************** 20.12 11 ************************************************************************************************* 19.5 12 ********************************************************************************************** 18.88 13 ******************************************************************************************* 18.26 14 ************************************************************************************* 17.02 15 ****************************************************************************** 15.78 16 ************************************************************************ 14.54 17 ****************************************************************** 13.3 18 ************************************************************ 12.06 19 ****************************************************** 10.82 20 *********************************************** 9.58 21 ***************************************** 8.34 22 *********************************** 7.1 23 ***************************** 5.86 24 *********************** 4.62 25 **************** 3.38 26 ************* 2.655 27 ********** 2.035 28 ******* 1.415 29 *** 0.795 30 0.175 Two weapon, level 7: 11.766 10 ************************************************************************************************************************************* 26.72 11 ********************************************************************************************************************************* 25.84 12 ************************************************************************************************************************ 24.08 13 *************************************************************************************************************** 22.32 14 ****************************************************************************************************** 20.56 15 ********************************************************************************************** 18.8 16 ************************************************************************************* 17.04 17 **************************************************************************** 15.28 18 ******************************************************************* 13.52 19 ********************************************************** 11.76 20 ************************************************** 10.0 21 ***************************************** 8.24000000001 22 ******************************** 6.48 23 *********************** 4.72 24 ****************** 3.72 25 ************** 2.84 26 ********* 1.96 27 ***** 1.08 28 * 0.2 29 0.08 30 0.08 Two handed, level 8: 11.34 10 ******************************************************************************************************* 20.74 11 **************************************************************************************************** 20.12 12 ************************************************************************************************* 19.5 13 ********************************************************************************************** 18.88 14 ******************************************************************************************* 18.26 15 ************************************************************************************* 17.02 16 ****************************************************************************** 15.78 17 ************************************************************************ 14.54 18 ****************************************************************** 13.3 19 ************************************************************ 12.06 20 ****************************************************** 10.82 21 *********************************************** 9.58 22 ***************************************** 8.34 23 *********************************** 7.1 24 ***************************** 5.86 25 *********************** 4.62 26 **************** 3.38 27 ************* 2.655 28 ********** 2.035 29 ******* 1.415 30 *** 0.795 Two weapon, level 8: 13.142 10 ***************************************************************************************************************************************** 27.6 11 ************************************************************************************************************************************* 26.72 12 ********************************************************************************************************************************* 25.84 13 ************************************************************************************************************************ 24.08 14 *************************************************************************************************************** 22.32 15 ****************************************************************************************************** 20.56 16 ********************************************************************************************** 18.8 17 ************************************************************************************* 17.04 18 **************************************************************************** 15.28 19 ******************************************************************* 13.52 20 ********************************************************** 11.76 21 ************************************************** 10.0 22 ***************************************** 8.24000000001 23 ******************************** 6.48 24 *********************** 4.72 25 ****************** 3.72 26 ************** 2.84 27 ********* 1.96 28 ***** 1.08 29 * 0.2 30 0.08
As you can see, first level still gives the advantage to the greatsword wielder. However, by level 2 (and the third feat) the two-weapon fighter has passed the greatsword, if only by a little. Then the advantage goes back to greatsword for very high AC foes. The greatsword holds that slight advantage until level 6, at which improved TWF gives the two-weapon fighter a significant advantage below 24 AC and a slight overall advantage. I only went up to level 8, since much past that and a no-feats greatsword wielder is at a pretty huge disadvantage. Level 10 opens up greater TWF and two-weapon rend, which cinch things.
Here are the numbers for paired short swords, in case you're interested. It's much worse, but not as bad as the guide above would have you think.
Two handed, level 1: 3.8955 10 *************************************************** 10.22 11 *********************************************** 9.5 12 ******************************************* 8.78 13 **************************************** 8.06 14 ************************************ 7.34 15 ********************************* 6.62 16 ***************************** 5.9 17 ************************* 5.18 18 ********************** 4.46 19 ****************** 3.74 20 *************** 3.02 21 *********** 2.3 22 ******* 1.58 23 **** 0.86 24 0.14 25 0.035 26 0.035 27 0.035 28 0.035 29 0.035 30 0.035 Two weapon, level 1: 2.913 10 ******************************************* 8.78 11 **************************************** 8.06 12 ************************************ 7.34 13 ********************************* 6.62 14 ***************************** 5.9 15 ************************* 5.18 16 ********************** 4.46 17 ****************** 3.74 18 *************** 3.02 19 *********** 2.3 20 ******* 1.58 21 **** 0.86 22 0.14 23 0.035 24 0.035 25 0.035 26 0.035 27 0.035 28 0.035 29 0.035 30 0.035 Two handed, level 2: 4.44075 10 ****************************************************** 10.94 11 *************************************************** 10.22 12 *********************************************** 9.5 13 ******************************************* 8.78 14 **************************************** 8.06 15 ************************************ 7.34 16 ********************************* 6.62 17 ***************************** 5.9 18 ************************* 5.18 19 ********************** 4.46 20 ****************** 3.74 21 *************** 3.02 22 *********** 2.3 23 ******* 1.58 24 **** 0.86 25 0.14 26 0.035 27 0.035 28 0.035 29 0.035 30 0.035 Two weapon, level 2: 3.84125 10 ****************************************************** 10.8 11 ************************************************* 9.98 12 ********************************************* 9.16 13 ***************************************** 8.34 14 ************************************* 7.52 15 ********************************* 6.7 16 ***************************** 5.88 17 ************************* 5.06 18 ********************* 4.24 19 ***************** 3.42 20 ************* 2.6 21 ******** 1.78 22 **** 0.96 23 0.14 24 0.035 25 0.035 26 0.035 27 0.035 28 0.035 29 0.035 30 0.035 Two handed, level 3: 5.022 10 ********************************************************** 11.66 11 ****************************************************** 10.94 12 *************************************************** 10.22 13 *********************************************** 9.5 14 ******************************************* 8.78 15 **************************************** 8.06 16 ************************************ 7.34 17 ********************************* 6.62 18 ***************************** 5.9 19 ************************* 5.18 20 ********************** 4.46 21 ****************** 3.74 22 *************** 3.02 23 *********** 2.3 24 ******* 1.58 25 **** 0.86 26 0.14 27 0.035 28 0.035 29 0.035 30 0.035 Two weapon, level 3: 4.4205 10 ********************************************************** 11.62 11 ****************************************************** 10.8 12 ************************************************* 9.98 13 ********************************************* 9.16 14 ***************************************** 8.34 15 ************************************* 7.52 16 ********************************* 6.7 17 ***************************** 5.88 18 ************************* 5.06 19 ********************* 4.24 20 ***************** 3.42 21 ************* 2.6 22 ******** 1.78 23 **** 0.96 24 0.14 25 0.035 26 0.035 27 0.035 28 0.035 29 0.035 30 0.035 Two handed, level 4: 5.63925 10 ************************************************************* 12.38 11 ********************************************************** 11.66 12 ****************************************************** 10.94 13 *************************************************** 10.22 14 *********************************************** 9.5 15 ******************************************* 8.78 16 **************************************** 8.06 17 ************************************ 7.34 18 ********************************* 6.62 19 ***************************** 5.9 20 ************************* 5.18 21 ********************** 4.46 22 ****************** 3.74 23 *************** 3.02 24 *********** 2.3 25 ******* 1.58 26 **** 0.86 27 0.14 28 0.035 29 0.035 30 0.035 Two weapon, level 4: 5.04075 10 ************************************************************** 12.44 11 ********************************************************** 11.62 12 ****************************************************** 10.8 13 ************************************************* 9.98 14 ********************************************* 9.16 15 ***************************************** 8.34 16 ************************************* 7.52 17 ********************************* 6.7 18 ***************************** 5.88 19 ************************* 5.06 20 ********************* 4.24 21 ***************** 3.42 22 ************* 2.6 23 ******** 1.78 24 **** 0.96 25 0.14 26 0.035 27 0.035 28 0.035 29 0.035 30 0.035 Two handed, level 5: 6.2565 10 ************************************************************* 12.38 11 ************************************************************* 12.38 12 ********************************************************** 11.66 13 ****************************************************** 10.94 14 *************************************************** 10.22 15 *********************************************** 9.5 16 ******************************************* 8.78 17 **************************************** 8.06 18 ************************************ 7.34 19 ********************************* 6.62 20 ***************************** 5.9 21 ************************* 5.18 22 ********************** 4.46 23 ****************** 3.74 24 *************** 3.02 25 *********** 2.3 26 ******* 1.58 27 **** 0.86 28 0.14 29 0.035 30 0.035 Two weapon, level 5: 5.702 10 ****************************************************************** 13.26 11 ************************************************************** 12.44 12 ********************************************************** 11.62 13 ****************************************************** 10.8 14 ************************************************* 9.98 15 ********************************************* 9.16 16 ***************************************** 8.34 17 ************************************* 7.52 18 ********************************* 6.7 19 ***************************** 5.88 20 ************************* 5.06 21 ********************* 4.24 22 ***************** 3.42 23 ************* 2.6 24 ******** 1.78 25 **** 0.96 26 0.14 27 0.035 28 0.035 29 0.035 30 0.035 Two handed, level 6: 9.30925 10 ************************************************************************************************* 19.5 11 ********************************************************************************************** 18.88 12 ******************************************************************************************* 18.26 13 ************************************************************************************* 17.02 14 ****************************************************************************** 15.78 15 ************************************************************************ 14.54 16 ****************************************************************** 13.3 17 ************************************************************ 12.06 18 ****************************************************** 10.82 19 *********************************************** 9.58 20 ***************************************** 8.34 21 *********************************** 7.1 22 ***************************** 5.86 23 *********************** 4.62 24 **************** 3.38 25 ************* 2.655 26 ********** 2.035 27 ******* 1.415 28 *** 0.795 29 0.175 30 0.07 Two weapon, level 6: 9.70725 10 ************************************************************************************************************************ 24.06 11 **************************************************************************************************************** 22.42 12 ******************************************************************************************************* 20.78 13 *********************************************************************************************** 19.14 14 *************************************************************************************** 17.5 15 ******************************************************************************* 15.86 16 *********************************************************************** 14.22 17 ************************************************************** 12.58 18 ****************************************************** 10.94 19 ********************************************** 9.3 20 ************************************** 7.66 21 ****************************** 6.02 22 ********************* 4.38 23 ***************** 3.455 24 ************* 2.635 25 ********* 1.815 26 **** 0.994999999999 27 0.175 28 0.07 29 0.07 30 0.07 Two handed, level 7: 10.31175 10 **************************************************************************************************** 20.12 11 ************************************************************************************************* 19.5 12 ********************************************************************************************** 18.88 13 ******************************************************************************************* 18.26 14 ************************************************************************************* 17.02 15 ****************************************************************************** 15.78 16 ************************************************************************ 14.54 17 ****************************************************************** 13.3 18 ************************************************************ 12.06 19 ****************************************************** 10.82 20 *********************************************** 9.58 21 ***************************************** 8.34 22 *********************************** 7.1 23 ***************************** 5.86 24 *********************** 4.62 25 **************** 3.38 26 ************* 2.655 27 ********** 2.035 28 ******* 1.415 29 *** 0.795 30 0.175 Two weapon, level 7: 10.94775 10 **************************************************************************************************************************** 24.88 11 ************************************************************************************************************************ 24.06 12 **************************************************************************************************************** 22.42 13 ******************************************************************************************************* 20.78 14 *********************************************************************************************** 19.14 15 *************************************************************************************** 17.5 16 ******************************************************************************* 15.86 17 *********************************************************************** 14.22 18 ************************************************************** 12.58 19 ****************************************************** 10.94 20 ********************************************** 9.3 21 ************************************** 7.66 22 ****************************** 6.02 23 ********************* 4.38 24 ***************** 3.455 25 ************* 2.635 26 ********* 1.815 27 **** 0.994999999999 28 0.175 29 0.07 30 0.07 Two handed, level 8: 11.34 10 ******************************************************************************************************* 20.74 11 **************************************************************************************************** 20.12 12 ************************************************************************************************* 19.5 13 ********************************************************************************************** 18.88 14 ******************************************************************************************* 18.26 15 ************************************************************************************* 17.02 16 ****************************************************************************** 15.78 17 ************************************************************************ 14.54 18 ****************************************************************** 13.3 19 ************************************************************ 12.06 20 ****************************************************** 10.82 21 *********************************************** 9.58 22 ***************************************** 8.34 23 *********************************** 7.1 24 ***************************** 5.86 25 *********************** 4.62 26 **************** 3.38 27 ************* 2.655 28 ********** 2.035 29 ******* 1.415 30 *** 0.795 Two weapon, level 8: 12.22925 10 ******************************************************************************************************************************** 25.7 11 **************************************************************************************************************************** 24.88 12 ************************************************************************************************************************ 24.06 13 **************************************************************************************************************** 22.42 14 ******************************************************************************************************* 20.78 15 *********************************************************************************************** 19.14 16 *************************************************************************************** 17.5 17 ******************************************************************************* 15.86 18 *********************************************************************** 14.22 19 ************************************************************** 12.58 20 ****************************************************** 10.94 21 ********************************************** 9.3 22 ************************************** 7.66 23 ****************************** 6.02 24 ********************* 4.38 25 ***************** 3.455 26 ************* 2.635 27 ********* 1.815 28 **** 0.994999999999 29 0.175 30 0.07