Edit6: Remade Post
You’ll need DirectX11 to do this, change the setting from DX12 to 11
I’m fully aware that WoW has no official CrossfireX support (which sucks!) but you can find the option to change what profile WoW uses for CrossfireX in AMD’s settings (Game>WoW>AMD Crossfire Mode>Use AMD predefined profile)
I tried all the Crossfire profiles AMD has (for other games, becnhmarks, etc)
I managed to get Crossfire working about as well as a single GPU using these profiles:
Hitman Pro (Hitman_CA.exe)
Rise of the Tomb Raider (TR2_x64_final.exe)
Sniper Elite 4 (SniperElite4_DX11.exe)
The Division (rogue_x64_release.exe)
Total War: Warhammer (Warhammer.exe)
Unigine Heaven Benchmark (Heaven.exe)
Obviously this isn’t ideal, but just wanted to let anyone trying to use Crossfire know that these ones do work about as well as a single GPU, which means you can now share the load between your 2 GPUs and get good results 
Worth noting that all of them have some flickering issues, but it’s hardly noticeable if you turn off depth effects and Frame Pacing.
Lowering MSAA also helps, though it obviously makes the game look worse, and turning off Multi-Sample Alpha Test gives a pretty big boost to FPS at no visual cost that I can notice
Let me know if you find any other settings that make a big difference in any way