The enhancement bonus would be the same as another fighter with one weapon. A manual of dex adds 2 points to your hit chance and the same manual is available to the guy with money to burn because he has a big sword instead of two little ones.
Whatever the outcome here, this money that the two weapon fighter saves in no way makes the two weapon fighter a better fighter, it just makes it so that the money available to one class for utility isn't diverted into the second weapon. It's the same as having a greatsword fighter in every way, they just don't have two years of a nations capital tied into there weapon choice. I don't know why the feat investment isn't enough of a tax on what amounts to flavor.
My apologies, it was late when I was typing. The manual does end up granting +2 to +3 modifier (depending on the stat being even or odd) not +5. Not as drastic as +5 but FAR from being dismissible. While the 2h might be able to get the bonus, the TWF is getting it on two weapons if not more (depending on feat choices etc.)
Feat tax isn't even an appropriate title now that PFRPG increased the number of feats you get versus 3.5, essentially they made it so whatever you wanted to do you had the feats to do it appropriately. Feats give you options, spend them how you like. When you have enough feats to do what you want and do it well, how is it a tax? Those were choices you made to have that option available.
This isn't "just" flavor. This is an ability that is so good it isn't an option to not take it at this and any similar price point. That is a problem mechanically. For it to be a fair option, the price point should be someplace around "this is good but there are other options I might want to spend the money on" and honestly that is probably around +3 if we are going to be conservative about it. Even +2 is pushing "must have" for any TWF type character.