I've tried switching power cables between both the GTX 460s and it produces the exact same results, so I'm pretty sure it's not the PSU. I've installed each GTX 460 alone in the top PCIe slot, and both appear to be working, although I haven't tried any serious gaming with the new PNY one. However, whenever I install both of them, only the Palit card shows up in Device Manager, regardless of what slots they are in. I've been all through the motherboard manual, and I can't find anything I haven't tried to get it working, so at this point I'm pretty sure something is broken. The question is, is it the motherboard or the GPU? I might be able to track down an SLI-capable motherboard to test the card, but none of my friends have SLI rigs, so if there's an easier way to tell that would be best.
Is the fan spinning on the PNY one? Have you enabled SLi appropriately in the nVidia CP?
Try putting the PNY GTX460 in one of your friends' machines to see if it works, if not then get a replacement.
The fan is spinning in the PNY one, and as I said, if it's the only card in the machine it will start up and work correctly. Well, it did once - I'm going to try it again when I get home from work just to make sure.
As far as I understand it, if the card isn't showing up in Device Manager, then the nVidia CP won't even allow you to set it up as SLI, or is there some other place?
Well I've done some further debuging and come to the conclusion that there's definitely someting odd going on with that specific card. I pulled out my old 8800 GTX and it works just fine with the Palit GTX 460, but not the PNY one. However, the PNY card works just fine if it's the only card installed.
I put in a support request to PNY to see if they have any idea what's going on and what they might suggest.