I have to test with gpuz to see if while running vr how the gpus are being utilized. A single 7950 has approx half the number of shaders as the r9 290.
With my dual 7950 vr was running amazing for what it is though reccommends medium settings found worked well with tweaking/choosing settings.
Just the odd thing with cf disabled no vr ap would launch in steam or piplay. Which according to old articles on sli & cf suggests that the games/aps should run with the single as most to none were not programmed with mgpu. The only thing i can figure is either if both gpus show utilization that
A) Amd has figured a way to have some form of cf support built into new driver.
B) the vrserver (steam/oculus/pimax) may have support built in as it manages stages & final output control to headset (headset driver)
C) Windows did something nice. Lol
With the prospect of mixed gpu support in future could be interesting. I remember a long while back folks figured out in Rage that a person with integrated nvidia gpu who installed a discrete amd card was able to enable cuda procesding while outputing on amd card. Nvidia fixed this of course in driver updates.