I'll say this first its been a long time since ive used SLI so things might have changed but, back when i did run multiple Graphics cards so long as the actual SLI was off then both cards would output signals but once the SLI was switched on then which ever card was running as the master card, only it would display and the other wouldn't.
Be aware that multiple graphics cards and SLI arn't the same thing. A computer can run multiple cards just fine but SLI/Xfire allows two cards to work together.
So you could run 5 monitors over two cards but once u turned on SLI one of the cards would stop outputing a video signal