I just had a thought. If I remove the once/round bit from Synchronized Strike, then a pair of Synchronized Swordsmen with high-dex and combat reflexes could set up a very nasty combo.

Let's assume they are both threatening the same target, and both have combat reflexes and dex 14, giving them 3 AoO's each per round. If a foe attacks one partner in melee, he provokes an AoO from the other. If that attack hits, the first partner (The one origionally attacked) ALSO gets an AoO, provoking another one from the second partner, and so on. With this setup (Which isn't unreasonable, considering it's a fighter based class), a pair of swordsmen attacked in melee would return one melee attack with six of their own. I don't know if that's overpowered, of if it's the sort of "Powerful Teamwork Ability" that Djinn was talking about.