Atleast I am pretty sure, On Gigabytes site they call it Hybird C/F and at the bottom they list.
(Note 3) If you are installing a PCI Express graphics card, be sure to install it in the PCIEX16_1 slot.
I know the 780 chip is nothing but a 770 chip with onboard.
But then again Asus has a 770 board with 2 PCIe X16 slots, but the second is only a PCIe X4 slot but they call it a CrossfireX board. But doesnt really explain how it does it.
I guess it could do one slot X16 and the other does X4, but thats 20 PCIe lanes, which is more than the 790X board in C/F which is X8-X8 in C/F, doesnt make any sense. I guess in crossfire it might knock down the first slot to X8 and the second runs at X4, but that would kill the performance of the second card.
Beats me!