You could run both of them, but they won't be SLI'd. You would only get the performance of the primary one when you play a game.
However, you could set the gt 630 as a dedicated PhysX processor, if you play any PhysX supported games. That should give a decent boost to fps, but it depends how well the gt 630 can process the physx stuff. It could potentially slow the 660 down. You would have to try it out I guess.
If you have multiple monitors, I think you can also plug in your extra monitors to the secondary card so the main GPU doesn't have to render the secondary screens.