Herobizkit
Adventurer
I wouldn't penalize someone for making an AoO after using a full-round action to perform a TWF attack. If said character was using a quarterstaff, he's get one swing with one end of the quarterstaff. If said character was using a dagger and a sword, he'd get to pick whether the dagger or the sword was... oh, yes, *now* I see the dilemma.
If the character used the weapon in his off-hand, I would induce off-hand penalties. If the character used his "main" weapon, no penalties.

If the character used the weapon in his off-hand, I would induce off-hand penalties. If the character used his "main" weapon, no penalties.