Okay we can even compare a normal twf fighter to a monk, say at lvl 12.
feats:
weapon focus, weapon spec, gweapon focus, gweapon spec, weapon mastery, twf, itwf, gtwf, two weapon rend, double hit, robillars gambit, combat reflexes
with the following equip, +4str/dex items, celestial armor, +3 amulet of natural armor, +3 ring of protection, 2 x rapier +1 +shocking +flaming
best race would be wood elf +2str/dex, -2con/int/cha
32 point buy
24str 18 base +2 from leveling +4 item
23dex 18 base +1 from leveling +4 item
12con
10int
10wis
6cha
at lvl 12 his attack pattern would be (12 base, +7str, +4wf,gwf,wm, +1magic weapon, -2twf)
22/22/17/17/12/12
damage for for the mainhand per swing:
1d6 +7 str +6 spec +1d6 electricity +1d6 fire (avg 23.5)
offhand per swing:
1d6 +3 str +6 spec +1d6 electricity +1d6 fire (avg 19.5)
for each time both weapons hit, you do another:
1d6 +10 damage (avg 13.5) from two weapon rend
AC 10 base, +6 dex +8 armor +3 natural armor +3 ring of protection (AC 30)
some cr 12-14 monster ac's
beholder ac20
95/95(to hit both 90%) , 85/85 (to hit both 72%), 60/60 (to hit both 36%)
first set of attacks avg 40.8 + rend 12.1
2nd set of attacks avg 36.5 + rend 9.72
3rd set of attacks avg 25.8 + rend 4.86
avg/round 129.78
average AoO dmg: 52.9 (which occurs every time the monster swings at him)
gelugon ac28
%70/70(to hit both 49%), 45/45 (to hit both 20%), 20/20 (to hit both 4%)
"" 30.1 + 6.6 rend
"" 19.3 + 2.7 rend
"" 8.6 + .5 rend
avg/round 67.8
average AoO dmg: 36.7 (which occurs every time the monster swings at him)
lvl 12 monk
same race
with the following equip, +4wis/dex/str items, bracers of armor +4, +3 ring of protection, +3 amulet of mighty fists
20str +4item, 16base
25dex +3 levels, +4item, 18base
12con
20wis +4item 16base
6int
6cha
AC 29
weapon focus, combat reflexes, (improved natural attack feat), weapon finesse, ??
attacks +9bab, +7dex, +1 weapon focus, +3 amulet of mighty fists
20, 20, 20, 15
damage per swing 3d6 + 5 + 3(amulet of mighty fists?) . average of 18.5 per swing, even if he hit with all 4 attacks 100% of the time, he would only be doing an average of 74dmg per round. Only slightly higher than the fighters ACTUAL average versus the high AC baddie. Actual damage would drop to about 40 damage per round. The suboptimal build for a fighter would do roughly 60% MORE damage, an optimal bulid... more than that.