The benefits from SLI (or crossfire) come when games specifically implement support for them, and they are generally assuming that you'd be running a single instance of the game. If you're running multiple monitors, you'd be better served by having them operating independently -- some on one, some on the other, and this would both improve performance (much like splitting instances between CPU cores) and stability for multiboxing. If you're not using multiple monitors, SLI might help but with multiple game instances, it may be unstable.