The HD 7970's that I've seen are double width GPUs, but some GTX 580's are triple width. You can fit up to 3-WAY double width on the ASUS P9X79 PRO. As for 2-WAY the two (Blue) PCIe slots are used, so you 'might' need to purchase a long/extended CF bridge BUT both PCIe x1 are covered so you'll need to use the (top White) PCIe x8 slot for the Sound Card with the bridge over its' top. This will likely cause the GPU's going from x16/x16 to x16/x8.
Very few X79's, if any I haven't seen one yet, have a top PCIe x1 slot above the 1st GPU PCIe slot. The reason is the Dual banks of DIMM slots on the right and left of the CPU on the LGA 2011.
If all you're going to do is 2-WAY then IMO you might want to consider either a Z68 or P67 the cost is significantly less and performance is very close. BTW - yes I realize the HD 7970 is PCIe 3.0, but the interface isn't going to make a difference over PCIe 2.0.