When playing Thief with Mantle enabled and using two GPUs in Crossfire, the game does not start. But if I have 3 GPUs enabled, I can play the game normally.
This is the only thing I can think of that could be causing this problem:
In MSI Afterburner and some other programs, the usage of my 4 GPUs looks like this (note that I have set the preference in the mainboard's BIOS of which GPU to use for the video signal output, which is set to PCIe 3 -the slot where the second graphic card would be- to PCIe 1, and I am also using this graphic card to output the video signal) :
1st is GPU2
2nd is GPU1
3rd is GPU3
4th is GPU4
I think what could be causing this is my mainboard. I am using an ASRock X79 Extreme11, which uses 2 PLX bridges for a higher amount of PCIe 3.0 lanes. This could be the problem, or some sort of traces left in the mainboard's BIOS from initially using PCIe3 for the video output, which was made alterable after a BIOS update.