You can react with your off hand to make an additional attack along with an attack of opportunity.

<php>echo “<b>Prerequisites:</b>”;</php> Combat Reflexes, Two-Weapon Fighting, BAB +6
<php>echo “<b>Benefit:</b>”;</php> When making an attack of opportunity, you may make an attack with your off hand against the same target at the same time. You must decide before your first attack roll whether you want to also use your off hand. If you do, both attacks take the standard penalties for fighting with two weapons.