You can attack with your off-hand weapon as frequently as with your primary weapon.
Prerequisites: Dex 25, Greater Two-Weapon Fighting, Improved Two-Weapon Fighting, Two-Weapon Fighting, base attack bonus +11.
Benefit: When making a full attack, you can make as many attacks with your off-hand weapon as with your primary weapon, using the same base attack bonus. For example, a character with this feat and a base attack bonus of +18/+13/+8/+3 could make four attacks per round with his primary weapon and four attacks per round with his off-hand weapon, using the same set of base attack bonuses. You still take the normal penalties for fighting with two weapons.
Normal: Without this feat, you can only make a single attack with an off-hand weapon during a full attack (or two attacks with an off-hand weapon if you have Improved TwoWeapon Fighting, or three attacks with an off-hand weapon if you have Greater Two-Weapon Fighting).
Source: Complete Warrior, page 152.