I have just built a new PC. The Motherboard is a ASUS P8P67 Deluxe and my graphics cards are two PNY Nvidia GTX 460s. My SLI worked wonderfully however, after installing a new heatsink for the CPU, for some strange reason, my computer only recognizes the one graphic card.
I am not sure if the Heatsink has to do with the problem. My second card does not show up in device manager, the computer acts as if theirs only one card. I tried each card in the first slot and they both work. I've tried turning the computer on with only one card in the second slot and it won't display. I guess it could be a bad slot but it worked perfectly prior to the heatsink.
What are some tests that I should perform to better troubleshoot the problem and find a fix. My OS is Windows 7 64bit. And I have tried reinstalling windows 7 and that did not fix it.
Those are the tests I would have done. It seems evident that both GPU's are working and PCI Slot 1 is functional. As the second slot does not respond (assuming to either card) then it looks like your motherboard has failed. It may have been damaged during the heat sink install or you may be just unlucky. Check your BIOS settings but to my mind the main suspect is your motherboard.