try a ' lspci | grep VGA ' you should have a VGA entry for each card that the machine sees. Or just ' lspci ' to see all pci devices.
If you only have one here, then the system isn't seeing it... check BIOS settings, try diff slots on the mobo if your using extenders. I've run into a board that will turn off x16 slots if it sees cards in certain slots, you'll have to go thu your mobo manual to be sure there.
If it does see both cards, then it's something in aticonfig .. one command I run all the time is ' aticonfig --odgt --adapter=all ' which should give you the temps of all the cards. If you have two in lspci, and only one using --adapter=all, then I'd double check the drivers and SDK.
I'd also try each card one at a time, to make sure you don't have a dead card..