It's definitely possible see
https://www.nicehash.com/index.jsp?p=news&id=155One of the M.2 is pcie only and the other can be pcie or sata is why only one has the option. I don't know why the 8th card causes a problem. It does sound like a pcie sharing problem so maybe more tinkering in the bios necessary. I assume your total power supply power is at least 1500?
Rig is running two CX750M power supplies with a 2psu adapter. All the risers + mobo + cpu + ssd are plugged into main PSU, and all cards are plugged into 2nd PSU. At the wall with 7 cards going, I'm only using like 800watts.
Been doing more digging... maybe its an IRQ issue? When I disable one random card in Device Manager, the other problem card will enable and work fine. IRQs on each card are: 17, 16, 18 ,16, 16, 19, 16, 16, so maybe too much on IRQ 16?