I have experimented with Xfire in Star Citizen now and then. Mostly it doesn't work as expected. Current patch has flickering textures in the PU, hasn't tested AC or SM much at all yet. I say expected as it is on DX11 still, there needs to be a profile from the Radeon team and some not insignificant work from CIG. So ofc it doesn't support it much. Vulkan will change things though, not only will it be much easier to make the Linux version, multi-adapter will work a lot different. Vulkan mGPU has the potential to be invisible to the developer, all GPUs are presented to the game engine as one. Potential that is, not something that is presently working IIRC.
Vulkan with Xfire is kinda of a pain at the moment. I have two Vulkan games, DOOM and Talos Principle. If Xfire is enabled, both games fails to start. DOOM just give me an infinite black loading screen, Talos gives me an error (same error in Linux BTW). If i turn off Xfire globally, then the games run. In Linux I have to unplug one of my cards (PowerColor), haven't found another way to make it work yet. I want to try the free Vulkan driver, radv, but haven't had time for it.
Another oddity is that DOOM runs on the second GPU in Vulkan mode. As the one that is not connected to the screen. It is kinda funny, First GPU (in the first PCIe slot too) stays cool while the second gets pegged to 100%. DOOM runs fine, I have tried to unplug the second GPU and I think the game runs the same on the primary. Haven't collected any data on it though, it would be kinda interesting to do. My second GPU is a bit odd as it is made by PoweColor (TUL Corporation). It had buggy BIOS out of the box and even after getting fixes from support it has never worked right. It basically doesn't work in Linux, I get whole system hangs etc. The primary card made by Sapphire has no problems, works in Linux just fine.
So Vulkan multi-adapter is not a thing yet. I have Ashes of the Singularity and DX12 multi-adapter works well in that game, looks like it works well in BF1 too.
