I really don't get the design of the PTXF feats.
Right now, THF have two feats to choose from. One of them gives 5% doublestrike. Ignore the fact that it says Perfect _TWO_ Weapon Fighting, it is actually best for THFers. The other is a decent bump in glancing blow damage.
TWF fighters have one feat... and that same feat is not as good as on THF fighters.
Doublestrike is one of the few weapon abilities that is more powerful for THFers than TWFers. 5% doublestrike on a THFer is nearly a 5% boost in damage. Sure, that goes down a bit with other sources of doublestrike, but even with an additional 20 doublestrike the feat is better than +4% damage. On a TWFer, that 5% doublestrike is only worth +2.7% damage... and that is with nothing to boost offhand attacks or doublestrike beyond the base 3 feats. Any additions just make it worse.
The main problem is that 10% offhand doublestrike is just a joke. As it only fires if mainhand doublestrike fires, it is only worth 10% of what it seems to be, and that assumes +5% doublestrike from another source. So all total you're looking at +3.7% tops.
I'm not sure the design goals of Turbine, but I can't imagine they planned something like this. Were they trying to make it so each feat is more or less +5% damage? While I'm not that positive at glancing blow details, back of the napkin calcs seem to imply that pTHF is also somewhere in the +5% range. (I'm curious what someone with more familiarity thinks.)
"Fixing" the PTWF so that the offhand is either +10% attacks or offhand doublestrike works with every offhand attack seems obvious.... but that just reverses the problem a bit. I guess maybe not that badly though.... Assuming offhand doublestrike meant that every offhand had X chance at doublestrike, 10 points would equate to 8 points of mainhand doublestrike (assuming nothing else going on). Together that would be an extra 13 attacks per 180 regular ones, or +7%ish. That would be better than a THFer gets from one feat (or not depending on my glancing blow skills), but less than a THFer would get from both together.
I'd like to see something done at least. Having PTWF be best on a THF toon just makes the whole thing look silly. As long as that aspect goes away, I'd find it reasonable.