Hmmmm,

2x MoBos
2x Video Cards
2x PSUs

Do you have, or know anyone who has an old conventional PCI video card? I would try it just to see, as there is no way there should be a power supply issue with that!

That leaves the CPU and the memory, can you borrow some memory or test it somewhere else?

That would leave the CPU, which I have a sneaking suspicion might be the culprit

What makes me say that is you are not getting any BIOS beeps. If it were the memory, you should at least get the memory error beeps. If it is the video, it should still get far enough into the post cyle to beep.

My money is on a dud CPU