The title says it all pretty much.
I did some research probably not good enough so I decided to ask the AMD community.
No,you can't,those gpus are from different performance leagues,also the 390 is very powerful anyway,why would you want to crossfire them?
Note:the ability to crossfire the gpus does not affect the ability to run the gpus independently on the same system.Your pc can use both gpus (separately) if you have some special usage scenarios (although,even if you go multimonitor the r9 supports six displays).
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