That shouldn't be any problem, do you have the latest BIOS?
Yes I have the latest BIOS loaded to my knowledge 2205. I tried just using one card in the first 16 slot and even alone it is x8 when the box says it should be x16. I hope I don't need a new mother board and I don't think it's power because I have 1300w PS
according to this the 2- x16 slots are wired to the cpu so the 3ed slot should not affect the 8x8 [always something to look at with intel boards] ??
is there a bios setting for each slot that may need change ? http://www.hardwaresecrets.com/article/ASUS-Z97-PRO-Motherboard/1860/2
Pcix 1 and 2 should run x16 at a single load but 1 runs at 8 and 2 runs at 4 and 3 runs at 1 which is not what the box says. I bought a new mother board that runs x16 in 4 lanes but I feel I gave up on this board too soon