I had a similar problem recently, and it was my PSU. Even though I had a 850W Antec 80+ bronze PSU. It went bad, and caused crashes. Both cards worked on their own, and both worked in either PCIe slot (make sure you tested each PCIe slot on their own).
It is possible the motherboard went bad if the cards won't work alone in the 2nd PCIe slot.
I tested both slots with th other card , everything is cool , The PSU is coming to mind again ( i was in doubt when i bought this couger psu ).