I believe this question has been asked before, and I believe the answer was no. I don't recall the exact reasoning, but I think it had to do with bandwidth limitation and priority in the system. Plus, as a second video card is usually only used for a dual monitor setup, and you wouldn't be able to run 2 different 3d applications at the same time.
Of course, I could be totally wrong, so don't take much stock in my answer.