I recently bought an AMD 990FX board in haste off of CL, because I didn't want to wait for shipping of an intel board. I could not get two Nvidia cards to work in it properly, no matter what I tried. In retrospect, I would have been much better off waiting for the intel motherboard via mail order.
Just my experience. One of the cards is a Dell branded GTX 1080. Maybe since it's an OEM card it has restricted compatibility?
The whole thing was a big run-around.
Sounds like you have a drivers or card issue, or didn't set up the BIOS on the MB correctly for multi-card.
I've got multiple NVidia cards working on a pair of 970FX boards - the 990FX is a higher-end chipset that should work at least as well.