Uh, TWF is much weaker. Compare a longsword and shortsword vs. a greatsword or Sword & Board on a 3rd level human fighter with 18 Str. The two swords guy has TWF, Power Attack, and Cleave. The greatsword/sword&board guys have Power Attack, Cleave, and Great Cleave. Assuming targets with AC16 and 2HD...
2WF: Longsword +5 (d8+4), Shortsword +5 (d6+2) Avg damage = 7.4375
2HF: Greatsword +7 (2d6+6) Avg damage = 17.991168
S&B: Longsword +7 (d8+4) (AC+2) Avg damage = 5.170176
(Avg damage assumes average die rolls (rounded up), accounts for hit probability, critical probability, and probability of killing target and getting a Cleave or Great Cleave facing four targets. For Sword & Board, it also factors in survivability due to added AC.)
Clearly, at 3rd level, 2HF is far superior to S&B, and blows 2WF away too. 2WF is marginally better than S&B, and the increased survivability of S&B probably balances nicely. 2HF, especially with a greatsword, is massively unbalanced.
Damn, I gotta quit taking 2WF at low levels...here I thought it was kickin', turns out it's chicken.
Hmm, what about at higher levels (11+)? Increased hit probability and number of attacks is going to tip the scales in favor of 2WF, but will it be enough with the 2HF & the S&B getting 2 more feats to devote to improving other areas? I think it will. I can't think of a feat selection that will offset the benefit of three extra attacks for the 2WF that either an S&B or a 2HF can pick. Shield bashing feats will bring the S&B up past with the 2HF, but the 2WF is sitting in the fabled catbird seat after 10th level. By this time, the poor 2HF had best start looking for a board, or another weapon... YMMV