If your problem is that your mobo doesn't recognize the gpus, maybe i have the solution.
Try to move your keyboard usb wire. I have a Alien keyboard with 2 usb, since I remove one of them, my mobo is recognizing all my gpus.
That's weird enough it might just work... I tried disconnecting my PS2 keyboard and USB mouse and still didn't get an POST, BUT the ethernet port was blinking differently so maybe there's something to it. Currently going through a power cycle to test that theory from a fresh state.
Considering sacrificing a goat in a satanic ritual just to get all the odds on my side, as well.