Well first thing to try is to uninstall the 2nd card, and see if the single top card works. If not, put that same card in the 2nd slot and try it. If it still doesn't work, and you've made absolutely certain everything is connected, then you probably have a dead card.
On the other hand, if it works in the 2nd slot, swap it out for the other card and do the same tests (top and bottom slots). If the top one doesn't work again, you have a bad PCIe lane on the mobo.
If both cards work on their own in both slots, make sure that you plug them in, power them both, and attach the crossfire bridge. Then you should only get video out of the top card.