I don't see why they should have synchronized strike. At the very least, it shouldn't be called that. It makes them attack twice as fast. From the name, you'd expect something more along the lines of being harder to block when they both attack.

Perhaps you should make it give them a bonus if they both attack the same opponent at the same time (the one that goes first would make a readied action to use this ability). This would be in addition to Improved Flanking.

You could change it to giving one an attack of opportunity by something other than attacking the other automatically gives an attack of opportunity to the other.

If one or both of them use ranged weapons, can they make an Attack of Opportunity form a distance? I think they should be able to, at lest with synchronized defense.