Your two-weapon fighting style bolsters your defense as well as your offense.
Prerequisites: Dex 15, Two-Weapon Fighting.
Benefit: When wielding a double weapon or two weapons (not including natural weapons or unarmed strikes), you gain a +1 shield bonus to your AC.
When you are fighting defensively or using the total defense action, this shield bonus increases to +2.
Special: A fighter may select Two-Weapon Defense as one of his fighter bonus feats.
Source: Player’s Handbook (version 3.5), page 102.