I did a bit of a study on the viability of TWF in preparation for a new campaign and I'd be happy to share what I learned.
I created a spreadsheet to track the math. It gets a bit hairy when you start trying to track critical damage and many iterative attacks.
My assumptions were as simple as I could make them while still trying to allow for a bit of "reality". I assumed that 20% of the targets would have an AC of the fighter's BAB+10. 40% would be below in 10% increments, and 40% above in 10% increments. So if you were first level with a BAB of 1, your AC spread would be 7/8/9/10/11/11/12/13/14/15. I recognize this is arbitrary, and that every game is different, but you have to start somewhere.
The fighter is human for the bonus feat, and put his +2 in strength. I modeled kukri and short swords throughout my level span comparisons. Kukri have the best crit ratio available (18-20 x2), while short swords have higher damage, while still being a light weapon. The idea was to not suffer the extra to-hit penalty of using a one-handed weapon in the off-hand, and to benefit from weapon focus, specialization and improved critical to both weapons.
What I found out is that short swords slightly beat out kukri in the first five levels, and that a two-handed weapon has parity... better under haste, but haste is the exception in the first five levels. Please note that my testing did not attempt to measure intangibles, and with more feats to spare, a two-handed fighter will have more intangibles to his advantage.
My assumption for the TWF for first fives levels were:
1st: TWF
1st: Double Slice
1st: Power Attack
2nd: Weapon Focus
3rd: Combat Reflexes
4th: Weapon Specialization
5th: Step Up
Kukri: 22.66 Short Swords: 23.46
I do not doubt that there were more optimum choices, and I invite criticism. The first level feats I think are optimum. Combat reflexes is an intangible, but with a high dex and some awesome awesomeness from the two-weapon warrior subclass... you betcha!
Moving into 6th-10th levels, the kukri zooms past the short swords as improved critical and critical focus kick in.
6th: ITWF
7th: Lunge
8th: Improved Critical
9th: Critical Focus
10th: Greater Weapon Focus
Kukri: 98.78 Short Swords: 90.96
From 11th-15th, Kukri establish dominance. Bleeding critical matters, unless I read it wrong, especially when you're critting plenty.
11th: Two Weapon Rend
12th: Greater Two Weapon Fighting
13th: Bleeding Critical
14th: Greater Weapon Specialization
15th: Blinding Critical
Kukri: 219.22 Short Swords: 207.98
My weapon assumptions were modest for magic. I didn't really try to follow through on a two-handed fighter comp because I didn't have the time to try optimizing feats for one, not that these are optimized for TWF.
Things that I learned from gaming the spreadsheet: crit is king starting level 8. Remember that x2 is the bottom, x3 is next, x4 and 19-20 x2 are the same, and 18-20 x2 is best. Would dual scimitars or rapiers be better at 11+ using Two Weapon Warrior? Yes, but the migration seems prohibitive. In addition, you're sacrificing 5% to-hit chance on both weapons, 10% later. Also, using a larger main hand weapon just doesn't pay off in the medium or long run, although you will benefit from it at first.
Haste, I think will propel two-handed weapons to parity or beyond once I have a chance to model them fairly. Right now, using rather crude feats, they are getting wooped. Strong items or buffs would just make it worse.
I didn't go beyond 15 because that doesn't happen often in the games we play. After a year you're ready to move on to a new campaign.
[edit] I was so impressed with the kukri, that I sent off for a nice one from Tora blade company in Nepal. Not cheap but wow... 14.6" of face-chopping awesome with a handle. If it's good enough for the chick in Resident Evil, it's.... sorry, what was I saying?
PS: if you try to model crits, remember that you have to model both chance of crit and crit confirmation chance when making the calculation. So a kukri at 8th level with a 50% to-hit chance has a 30% chance of critting (15-20) x 50% chance of confirmation = 15% crit damage add per hit for full crit damage. At 9th level with critical focus, it's 30% crit chance x 70% confirm chance = 21% crit damage add per hit. Less with iterative hits and less damage to off-hand due to power attack, so each has to be calculated individually.