Have completely read the manual for your motherboard and it doesn't discuss running in Crossfire X other than your second card slot runs at x4. There is no supplementary power connection to provide additional power to the second card. Revision version is in lower left corner.
Study of your cards indicates there are two GPU BIOS options. Have you made sure they are BOTH in the same positions? Have you updated the BIOS in the cards? They are almost 1 1/2 year old releases. Using one Crossfire bridge? Is the voltage reading in Sapphire TRIXX, Afterburner, etc. as stated in the specs for your cards? Have you swapped the cards to the opposite expansion slots?
Your cards operate at 250 watts (load) each. Have you checked in Windows to be sure that your Power settings are in High Performance mode?
Seems with a black screen there is a power issue but as stumped mention it could be the drivers if one card is running perfectly...swapped also, correct?
Those are powerful cards and they need a better motherboard with a 990 chipset to get the best out of your system...see ASUS, GIGABYTE, etc, high performance versions for outstanding results. Better off getting a new mobo than getting rid of one of your cards!
Highly recommend you get more RAM...8gb to 16gb total at modest speed. Looks like you have sufficient enough PSU.