Go to the ASUS website and download a copy of the manual for that motherboard! Read through the UEFI settings that pertain to onboard devices setup! I have the P9X79 Deluxe model and the PCIe slots are shared with other onboard functionality, such as eSATA or BlueTooth! You have to make a choice of whether you can use the slot for a card, or the slot is disabled in favor of the other function. You may not have those particular options on your motherboard, but ASUS may have assigned double duty, or either/or options for that motherboard as well! So two graphics cards… as You and Scott suggested, may very well be causing a resource problem!
Just remove one of the graphics cards for a test! But, start with the BIOS settings because ASUS will hide the motherboard limitations in the fine print! 
Al