Synastry is a bit op.

[Remilia_Nightfall]

Is it powerful? Yes.
Is it OP? No.

It's a nice tool for AST to have, one that actually plays the support role nicely since it allows the other healer to go DPS for a while when both tanks are tanking something.

I think he is trying to play it off as 100% of heal on tank A and 50% heal on tank B equals 150% healing achieved.

While technically right, a good AST will go beyond that.

Place Synastry and AoE heal.