I just installed a second R7 370 card today and now Windows 10 will not shut down or restart when Crossfire is enabled. Windows will load if i power down the PC and power it back on. I deinstalled CCC/drivers (ver 15.30.1025) and reinstalled without any success.
MSI X99A SLI PLUS MOBO
32 GB Ribjaws 4 DDR4 SDRAM
Corsair AX860i Power supply
Not sure what other system information you may need.
I also found that the fans on my GPU go 100% when I enable Crossfire.