Note: This question has been answered for D&D 4e, but I'm asking if this also works in D&D 3.5 since the systems tend to differ significantly over different editions. ( Does a Double Weapon work with Two-weapon feats? )
So can a Two-weapon feat be used with a double weapon in 3.5? Same question also goes for the feat "Ambidexterity".
For the purposes of the Two-Weapon Defense feat in particular,
Two-Weapon Defense [General]
[...]
Benefit: When wielding a double weapon [...] you gain a +1 shield bonus to your AC.
It doesn’t get any more explicit than that.
Ambidexterity was a feat in 3e, which was removed from the game in 3.5e. It is no longer necessary at all; its benefits are just the default for 3.5e characters (the penalties for two-weapon fighting were larger in 3e).
As for other “Two-Weapon” feats, well, they can be explicit about it, like Two-Weapon Defense is. Or they can piggyback off of the general two-weapon fighting rules, as Two-Weapon Fighting, Improved Two-Weapon Fighting, and Greater Two-Weapon Fighting do. Or they can be... a semi-undefined case.
For the sake of clarity, note that above I referred to “two-weapon fighting” and “Two-Weapon Fighting.” A terrible choice on Wizards’ part, I’m afraid, to name things so confusingly. When I say “two-weapon fighitng,” I am referring to the two-weapon fighting specal attack that anyone can use. When I say Two-Weapon Fighting, I am referring to the Two-Weapon Fighting feat you can take to lower the attack penalties when using the two-weapon fighting special attack. So, the special attack is in normal case (capitalized at the beginning of a sentence and otherwise not), and the feat is in title case (each word capitalized).
Now then, about double weapons, two-weapon fighting, and Two-Weapon Fighting.
Strictly speaking, double weapons count as a paired one-handed weapon and light weapon for exactly two purposes:
Allowing you to use the two-weapon fighting combat option, treating one end of the double weapon as offhand, and
The attack penalties you take when you choose to use that option.
Note that #2 covers Two-Weapon Fighting—it talks about the attack penalties you get when you use two-weapon fighting, for which the double weapon count as paired one-handed and light weapons. And #1 takes care of Improved Two-Weapon Fighting and Greater Two-Weapon Fighting, as those talk about your attacks with your “offhand weapon,” which is one end of your double weapon per the double weapon rules for two-weapon fighting.
For everything aside from those two points, however, and this is being incredibly nit-picky and, frankly, obtuse, the double weapon does not count as a one-handed weapon paired with a light weapon—it counts as what it is, which is a two-handed weapon. Including for, say, damage, despite the fact that the rules explicitly highlight the option of not using two-weapon fighting in order to get the two-handed weapon damage, and the fact that a prestige class (revenant blade from Player’s Guide to Eberron) exists that has the ability to treat each end of a double weapon as a two-handed weapon for damage as its capstone feature.
This was almost-certainly an oversight. When you are wielding a double weapon and invoking the two-weapon fighting special attack, you should be treating the ends of the double weapon as a one-handed weapon and a light weapon, respectively. That means that even if a feat fails to specifically say something about double weapons, but instead discusses a pair of weapons, you should feel comfortable letting a double weapon qualify, at least while using the two-weapon fighting special attack. Likewise, things that specify attacking with a two-handed weapon should not apply to a double weapon when using the two-weapon fighting special attack (unless you’re a 5th-level revenant blade, of course). But, strict RAW, you could argue for the opposite: things for two-handed weapons work with double weapons even while using them for two-weapon fighting, but things for a pair of weapons might not work for double weapons (depending on whether they fall under #1 and/or #2, above, or not), even when using them for two-weapon fighting. This is not recommended.
