you need to file a bugs report with amd I also have this issue " I run two r9 295x2 " but thought it was my free freesync " my monitor had to go into rma and asked to have it added and they did , benq is awesome" , now with the oc I don't have that issue but my monitor is only 60 hz " 4k " I run out of psu before I blackscreen or get an unstable oc .
i have same issue. freesync without crossfire works great.
if i enable crossfire huge fps drops sometimes and game looks not stable.
i locked my gpu clock to max its work fps drop issue on single gpu. but in crossfire fps still drops.