I think I tried this once on a Gigabyte P45 board..... supports xfire only. And I think it worked okay. I believe I installed the main card, installed the drivers, shut down, rebooted after installing 2nd card. I think I reinstalled the physx only driver from nvidias site. Then went into control panel to select the right card for physx.
wait one........ installed both cards. One the monitor was hooked to became main card. I think it was already selected ( second card ) as physx in the control panel. It won't hurt anything to try. Not like it will fry anything.