Okay Im trying to Xfire a 280x probs

Okay so i have a 280x and a 7970ghz oc edition im trying to crossfire them but when i turn on my pc no display but the boot sounds over speakers and it even logs into my Desktop but i cant get a display i have a decent rig, 6300 sixcore amd, 850 watt Bronze cert psu, Help please. 

try one card at a time make sure they both aren't dead and then if they arent then swap them round and give it a whack, might be a mobo problem but i dont think so, let me know whats up