You gain a significant defensive advantage while fighting with two weapons.

<php>echo “<b>Prerequisites:</b>”;</php> Dex 17, Two-Weapon Defense, Two-Weapon Fighting, BAB +6
<php>echo “<b>Benefit:</b>”;</php> When wielding a double weapon or two weapons (not including natural weapons or unarmed strikes), you gain a +2 shield bonus to your AC. This bonus stacks with the shield bonus granted from weapons with the defensive property. When you are fighting defensively or using the total defense action, this shield bonus increases to +4.