Harrying Partners (Combat, Teamwork)Source Melee Tactics Toolbox pg. 10 (Amazon)
Your movements perfectly synchronize with your ally’s to greater effect.
Prerequisites: Any teamwork feat, base attack bonus +6.
Benefit: When you successfully use the aid another action to improve the Armor Class or attack roll of an ally who also has this feat, the benefit from aid another lasts until the beginning of your next turn.
Normal: The bonus granted by aid another either grants your ally a +2 bonus on her next attack roll against an opponent or grants your ally a +2 bonus to AC against that opponent’s next attack made before the beginning of your next turn.Source Armor Master's Handbook pg. 20 (Amazon)
When using the aid another action to help an ally who also has this feat, you can spend 5 stamina points to grant a +4 bonus instead of the normal +2. Alternatively, you can grant your ally a +2 bonus both on her next attack roll and to her AC against the next attack against her. (Melee Tactics Toolbox 10)