In order to be good at fighting with a two-handed weapon, one merely has to pick one up and have a high Str score.
See and right there is what you're doing wrong.
What you NEED to do is learn how to play a Rogue
before you run off complaining about it.
What level are these characters supposed to be? 10? Your numbers are wrong (the THF Barb's damage is actually 268) and fail to account for the fact that even DR 5 brings the TWF Barb down to 276. The TWF Barb is also 10% less accurate with each of its attacks. Unless they're both off the RNG, that means the TWF Barb deals 10% less damage, bringing it down to 285 (or 249 with DR 5).
A. 2d6 (avg 7)+13 +40 = 60, times four for the number of attacks is 240. Not 268, you failed your math check not me.
B. Do you even understand what you are trying to say? Hits a Couatl on a 2, hits a Bebilith on a 3, hits a Fire Giant on a 3, hits a Clay Golem on a 2, hits an 11 headed Hydra on a 2, hits a Rakshasa on a 2. Most of those are because a 1 always fails. 10% less accurate is of ZERO VALUE if your hit rate is still pretty much capped to begin with.
C. None of the models acknowledge DR until now. Not my fault you didn't think of it before and are now grasping at it.
Okay? So, it doesn't make Rogues/Factotums/Bards less capable of using TWF well. It gives new toys to pure melees. But I guess that's forbidden.
No it doesn't I said that before.
Pure melees take melee feats, spellcasters take spellcasting feats. It's why melees would consider investing three feats into TWF for three additional attacks. You've dropped that in favor of a Wizard casting Herorics on him self to double his attacks. This is NOT a melee buff, this is making things easier for casters to simulate the melee rule with less effort than before.
Secondly, specialist classes derive their damage output from TWF. Like a Rogue would seek to deal +20d6+20, or +90 avg, over six or more attacks sporting a HIGHER damage rate than a tank. To balance this they have a less reliable hit chance and less viability. Core fundamental is less aim = more damage and can be observed in PA, TWF, Furry of Blows, etc, and in thousands of games outside of D&D. You went and gone to upset this by awarding bonus damage to the tank to bring them closer to the specialists. Combined with all casters picking up Full TWF, the specialist's nich is significantly reduced. The problem is, a specialist is still melee. You've debuffed the T3 classes to empower the T4/T5 classes and now everyone is T4. This is NOT a melee buff, this is a sloppy and overpowered award that depreciates the very people you wish to help.
Most players don't hit with every attack, every round. Yes, it's possible. With ease. But most people just don't do it. At least in my experience. Building an Uber-Charger is spectacularly easy too, but again, in my experience, most people just don't. In the games that I've actually played the save-or-suck effects of these feats require significant investment and won't happen every round.
As demonstrated, the TWF Barb mostly only misses on a natural 1, you know, the always miss 1. So explain this most players don't hit if the examples always do.
The aforementioned Warblade's Feral Deathblow should have made you stop and think longer than thirty seconds. The ToB commands a lot of respect on these forums and is frequently banned for being too powerful. It awards a single save or die effect to 17th level characters as part of a SINGLE attack and can normally be done at best once every two rounds. Rather than dwelling on this you went with level 12 and once per four attacks. No, minor correction there. Once per two attacks since TWF double things, like even a 5th level Wizard would have four attacks per round.
TLDR; what we're all tell you is it's insanely powerful.