Sorry to announce that, but your calculations are more than a little wrong:
one d20 chance to hit= P
2 d20 chace to hit twice (at -3 penalty)= Q^2
2 d20 chance to hit at least once
= 1-(1-Q)^2 = 2Q-Q^2
where
Q=P-2/20 (penalty of TWF) -1/20 (no weapon focus)=P-3/20
average hit per round= 2Q
Now if Q=10/20 you indeed have a an "effective" +10 to hit, in fact if Q=x/20 you have "effectively" +x to hit. Now let P=y/20 then the bonus relative to the 1 weapon guy is
[2(y-3)-y]/20=(y-6)/20
(Note that this does not take account of the fact that you always hit on 20). So you have actually cases where the you have more chances to hit with one weapon only to the contrary of the "+10 to hit" (very) crude approximation.
Now let's look at the damage (without crit):
Prob to hit with 1 attack=P
Average dmg 1 weapon: Dmg*P
so in the case of the 2HF Dmg=7+2(WpnSpec)+1.5Str
Average dmg 2 weapons:
(Dmg1+Str)*(P-3)+(Dmg2+.5Str)*(P-3)
= (Dmg1+Dmg2+1.5Str)*(P-3)
with Dmg1=4.5 and Dmg2=3.5 so
Now Avrg dmg 2WF= (8+1.5Str)*(P-3)
which is actually only worse than the 2HF! Of course you could make some calculations and find that actually if their weapon is masterwork, and with a *perfect* use of power attack your 2HF is better than the 2WF for AC 7 to 28, which at this level is almost any opponent; of course your cases were under-optimal, you could design a more advantageous case for the TWF (though never have a always better TWF case), the main problem is that it is never quite worth the 3 or 4 feats you paid, and I don't feel an optionnal prestige class to be such a good fix. As JLXC noted, giving full-Str bonus on off-hand is the ideal fix you need: it simplifies combat and does not unbalance styles.
As a general remark one of the very big error comes from the simplificative use you make of power attack: a 2WF who uses badly his power attack loses less in average dmg than a simple weapon fighter that uses it badly.
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Finally, for those who are interested in comparing 2WF and 2HF, I already posted on this site one of my excel worksheet (with macros) which makes a nice table that gives you average dmg for Mr.1, Mr.2 and the difference between them. It is flexible enough so you can modify many things like Str bonus multiplier to dmg, 2 weapon fighting penalties, etc...
I won't attach it again here, since I fear to slow the boards, but it is in the thread
"Two weapon Wackiness", started by Xarlen, last post by Spikey Freak on Feb-05-2002, the link is
http://www.enworld.org/messageboards/showthread.php?s=&threadid=2293
attached to one of my posts (I think on the second page). It includes a .txt readme to explain how it works. What is the price you ask??? well it is free!!! Incredible! and you can tinker with it all you want since I don't have time to do it myself...
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Xeovke
BTW, for those who were on the old ENworld boards I am sorry about not being able to continue that discussion, but I was busy for some time (and I am still too much to get involve in long discussions) and the boards closed.
Ignoring strenght bonus is in favor of the 2WF since he has 1.5 Str placed at the +4BAB-2Penalty=+2, while the 2HF has a 1.5 placed at +4BAB+1WeapFoc=+5. Ignoring critical hits does not make a great difference overall, and in the cases you mentionned it actually advantages the 2WF (by some .1 average dmg per round).Mustrum_Ridcully said:
Ummm.... not really, in fact, not at all:
one d20 chance to hit= P
2 d20 chace to hit twice (at -3 penalty)= Q^2
2 d20 chance to hit at least once
= 1-(1-Q)^2 = 2Q-Q^2
where
Q=P-2/20 (penalty of TWF) -1/20 (no weapon focus)=P-3/20
average hit per round= 2Q
Now if Q=10/20 you indeed have a an "effective" +10 to hit, in fact if Q=x/20 you have "effectively" +x to hit. Now let P=y/20 then the bonus relative to the 1 weapon guy is
[2(y-3)-y]/20=(y-6)/20
(Note that this does not take account of the fact that you always hit on 20). So you have actually cases where the you have more chances to hit with one weapon only to the contrary of the "+10 to hit" (very) crude approximation.
Now let's look at the damage (without crit):
Prob to hit with 1 attack=P
Average dmg 1 weapon: Dmg*P
so in the case of the 2HF Dmg=7+2(WpnSpec)+1.5Str
Average dmg 2 weapons:
(Dmg1+Str)*(P-3)+(Dmg2+.5Str)*(P-3)
= (Dmg1+Dmg2+1.5Str)*(P-3)
with Dmg1=4.5 and Dmg2=3.5 so
Now Avrg dmg 2WF= (8+1.5Str)*(P-3)
which is actually only worse than the 2HF! Of course you could make some calculations and find that actually if their weapon is masterwork, and with a *perfect* use of power attack your 2HF is better than the 2WF for AC 7 to 28, which at this level is almost any opponent; of course your cases were under-optimal, you could design a more advantageous case for the TWF (though never have a always better TWF case), the main problem is that it is never quite worth the 3 or 4 feats you paid, and I don't feel an optionnal prestige class to be such a good fix. As JLXC noted, giving full-Str bonus on off-hand is the ideal fix you need: it simplifies combat and does not unbalance styles.
As a general remark one of the very big error comes from the simplificative use you make of power attack: a 2WF who uses badly his power attack loses less in average dmg than a simple weapon fighter that uses it badly.
[Ad mode on, start groovy music]
Finally, for those who are interested in comparing 2WF and 2HF, I already posted on this site one of my excel worksheet (with macros) which makes a nice table that gives you average dmg for Mr.1, Mr.2 and the difference between them. It is flexible enough so you can modify many things like Str bonus multiplier to dmg, 2 weapon fighting penalties, etc...
I won't attach it again here, since I fear to slow the boards, but it is in the thread
"Two weapon Wackiness", started by Xarlen, last post by Spikey Freak on Feb-05-2002, the link is
http://www.enworld.org/messageboards/showthread.php?s=&threadid=2293
attached to one of my posts (I think on the second page). It includes a .txt readme to explain how it works. What is the price you ask??? well it is free!!! Incredible! and you can tinker with it all you want since I don't have time to do it myself...
[Ad mode off, end groovy music]
Xeovke
BTW, for those who were on the old ENworld boards I am sorry about not being able to continue that discussion, but I was busy for some time (and I am still too much to get involve in long discussions) and the boards closed.
Xeovk, don't tell me I missed the cow in the avatar list and you got it first! 
