In my experience Synergy is a little wonky. It gets you about 90% of the way to what you want.

However, for gaming I would think it would be tough - I would think it would be easy to accidentally switch back to the other machine in the middle of the game.

You can hit scroll lock to lock the mouse to 1 screen. As long as you game on the synergy server machine there is no problems.

Just make sure your inputs are connected to the machine you do gaming on. If you do gaming on both, then this might not be an option.