I stopped at 6th because that's all it took to get ITWF...so basically by then you have 3 gravy feats. You can even focus/spec in your shield bash instead of the shield spec/ward if you want to go more offensive.
Bagpuss would be spot on except the off hand won't get any added PA damage.
2H vs. TWF...it's the price you pay. In all honesty TWF totally sucks for people who don't have extra damage dice like sneak attack EXCEPT shield bashers. Yeah, greatsword guy gets all those bonuses, but he doesn't get oh...+5 AC from a decked out heavy shield. It's not 2d6+7 vs. greatsword...it's 2d6+7 + 5 AC vs. the greatsword
A few things though...light shields don't get PA damage, and heavy shields are one-handed...so that carries a -4/-4 penalty. I've never understood the penalty in the first place TBH...if you are trained to TWF why are you taking penalties? It's really not *that* difficult that all humans just inherently suck at it; when I was a student in kung fu there were several guys that were proficient in dual wielding the chinese broadsword, and in weapons sparring, it was more difficult to fight them, because they had...two damn swords.
TWF is nice for those who have sneak attack style damage. They aren't worried about PA though, because they tend to have balls for STR stats and even if they don't they're taking things like WF and II.
So basically if you want the PA damage of a 2H you have to wield 1H (not light) weapons and you take -4/-4; this is after taking an extra feat every 6 levels. At that point you are doing the same damage...but that's if both of your gimped attacks hit.
Fighter A with a +2 greatsword and 18 str vs. Fighter B with +1 longswords and 18 str, using 5 PA. Both have specialization.
Like a +12 / 2d6+20 vs. +8 / 1d8+12 and +8 / 1d8+9
So after using an extra feat and having a high dex score that you really don't need (and thus likely lowering your Con and even Str), you're doing 2 points higher on average as the 2H fighter...if you hit twice with -4 to hit and wield identical weapons.
Wow, that does suck.
Just get an animated heavy shield and greatsword people in the face.