Your two-weapon fighting style bolsters your defence as well as your offence.
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. See the Two-Weapon Fighting special attack.
When you are fighting defensively or using the total defence action, this shield bonus increases to +2.
Special: A Fighter may select Two-Weapon Defence as one of his fighter bonus feats.
Source: Player’s Handbook (Page 102)