You gain a significant defensive advantage while fighting with two weapons.
Prerequisites: Dex 17, Two-Weapon Defense, Two-Weapon Fighting, base attack bonus +6.
Benefit: When wielding two weapons (not including natural weapons or unarmed strikes), you gain a +2 shield bonus to your Armor Class.
When you are fighting defensively or using the total defense action, this shield bonus increases to +4.
Special: A fighter may select Improved Two-Weapon Defense as one of his fighter bonus feats.
Source: Complete Warrior, page 101.