I'm not exactly sure how these would stack if they were both allowed. Would the -2/-2 with a light weapon and TWF be eliminated or would it be exactly the same (thus making Ambidexterity obviously redundant)?
The 3.5 Two Weapon Fighting feat already includes the benefit of 3.0 Ambidexterity.
3.0 Two Weapon Fighting penalties:
If a normal character (i.e. a character without the multiattack and/or multidexterity feats) wields a second weapon (fights two handed), that character can get one extra attack per round with that weapon. Fighting in this way is very hard, however, and the character suffers a -6 penalty for regular attacks with the first hand and a -10 penalty to the attack with a other hand. A character can reduce these stiff penalties in three ways:
- If a character's second weapon is light, the penalties are reduced by 2 each.
- The Ambidexterity feat reduces the penalty against the second weapon by 4.
- The Two-Weapon Fighting feat reduces both penalties by 2.
3.5 Two Weapon Fighting penalties:
If you wield a second weapon in your off hand, you can get one extra attack per round with that weapon. You suffer a -6 penalty with your regular attack or attacks with your primary hand and a -10 penalty to the attack with your off hand when you fight this way. You can reduce these penalties in two ways:
- If your off-hand weapon is light, the penalties are reduced by 2 each. (An unarmed strike is always considered light.)
- The Two-Weapon Fighting feat lessens the primary hand penalty by 2, and the off-hand penalty by 6.
Presented in a table for comparison
PRI OFF PRI OFF
No feats -6 -10 -6 -10
Light weapon -4 -8 -4 -8
+TWF feat -2 -6 -2 -2
+Ambidex feat -2 -2 N/A N/A
Ambidexterity (by common English definition) means your off hand is as good as your primary hand. If you tried to reintroduce the 3.0 feat exactly as written, then it would make your off hand better than your primary hand in 3.5.
If you were to reintroduce Ambidexterity into 3.5, then the whole of 3.0's two weapon penalty table should be reintroduced with it - ie, the Two Weapon Fighting feat would be far less effective at improving your off-hand attacks.
They do not stack at all.
Ambidexterous not appearing in 3.5 is not a case of oversight or cutting content to save space. You have to remember that the entire purpose of 3.5 was to revise a significant number of design defects in 3.0. In 3.0, any character who wanted to take Two-Weapon Fighting was functionally guaranteed to take Ambidexterous, which led to heavy abuse of single-level Rangers. Because of this, the designers opted to roll part of the benefit of Ambidexterous into Two-Weapon Fighting and do away with the first feat entirely.
If you want to introduce ambidexterity back in I would say 2 weapon fighting would be a prerequisite. It would further decrease the penalties for the weapons by -2/-2 thus nullifying any penalties for using 2 weapons/ 2 handed weapons. You only get so many feats in the run of a game (assuming you stop at 20th level).