I have two RX 480 Red Devil Power Color cards in my system. Basically, I've tested these graphics cards independently, they both work GREAT! However when I operate them together, the card that I put in PCI-E Slot 2 doesn't work. Here's the trouble shooting I've already done:
1. Swapped Motherboard. (First motherboard DID have an issue, the first PCI-E slot would cause blue screens, this one works GREAT.)
2. Swapped out graphics cards and tested both slots.
3. Uninstalled and Re-installed Graphics Drivers*
4. Wiped Graphics Drivers with 3dGuru's driver uninstaller.
5. Installed Graphics again. (Everything worked fine both graphics cards detected, until the next restart.)
6. Swapped RAM (Unrelated).
* While I re-install graphics, the monitors that normally do not work, the ones hooked to the second graphics cards, do turn on while the install is occuring. To the point that I can adjust them with screen resolution in windows.
Asus Sabertooth Mark 1 / USB 3.1
Intel i5-4690K 3.50GHz
32 Gigs of 1600 MHz Kingston ram
1 TB Micron m500 SSD