Did you ever get more than 2 cards to work on this board? I am having the same problem.
Enabling the IOMMU in BIOS worked for me with the same MOBO. But I couldn't find the IOMMU! So for anyone else looking... It's under the OC Menu, then look at CPU features.
That didnt work for me. I ended up RMA the board. I have a gigabyte coming in today.
Thanks for the reply though.