Made one of these on my server a couple weeks ago, except that both players stand on opposite sides, and have their own set of 8 buttons. It's a little easier to play without having to flip the switch between each turn. Also, at the bottom is another row of pistons over a trench filled with torches. A button retracts the pistons and drops all the blocks into the trench, where they're destroyed by the torches for easy retrieval.
Next step will be to add circuitry to prevent each player from going until the other player goes, in order to enforce the turn order. Shouldn't be too complicated, but will require a lot of additional excavation (I built it in a huge cavern just above bedrock) that I haven't gotten around to yet.