So, I've been having some problems with getting SLI to work and I think I've narrowed it down. I ended up taking my build completely apart to start over from scratch and it seems I bent some pins in the cpu socket when I first put it together. The system boots and runs fine, it won't recognize the second video card but won't run if both cards aren't plugged in. My question is whether or not the bent pin could be the cause of this.
It's certainly possible, on the current Intel CPUs the PCI-E controller for the video card slots is on the CPU. A bent socket pin might disable one of the slots. Either replace the motherboard, or if you have very fine motor skills try to straighten the bent pin.