SRD3e:Improved Two-Weapon Fighting
From D&D Wiki
You are an expert in fighting two-handed.
Prerequisites: Two-Weapon Fighting, Ambidexterity, base attack bonus +9 or higher.
Benefit: In addition to the standard single extra attack you get with an off-hand weapon, you get a second attack with the off-hand weapon, albeit at a -5 penalty.
Normal: Without this feat, you can only get a single extra attack with an off-hand weapon.
Special: A ranger who meets only the base attack bonus prerequisite can gain this feat, but only when wearing light or no armor.