This is NOT the recommended method, but I've read at least someone had it working.
Plug your monitor in to the bottom card. The top card in SLI works in single gpu situations because that is where the monitor , memory buffer is connected.
If your system boots, I'm pretty sure your #2 card will be the active card in non-sli modes.
edit: I have no temp problems, and my top card does run cooler than when they are both going. I guess I don't think this is worth exploring, imho.