Star Citizen has not been released yet, so it does not have a CrossFire profile.
Have you tried using the Crysis 3 profile?
I have actually, and monitoring software shows no activity from 2nd GPU when Crysis 3 profile is selected (honestly, its barely even CryEngine anymore its so heavily modified).
The Crossfire "Modes" do actually work, but only AFR Friendly gives better performance over single GPU but as mentioned in OP, it causes the "black frame" issue. Im trying to find out if it is a defect with my 2nd GPU, which I think it might be because I just tried the SteamVR Performance Test and the right-eye screen is having some graphical issues that appear to be linked directly to the GPU usage. I say this because GPU1 (rendering left-eye screen) is at a consistent 100% with no graphic errors while GPU2 usage drops frequently and coincides with the graphic errors im seeing (not quite like artifacting but not too different either).
Edit: Uploaded a short video of the SteamVR test issue