I'm in the same boat, here's the problem:
1.) AMD's WoW crossfire profile only works for DX11, because the low level control necessary to make multiple GPUs (mGPU) appear as one to the game engine, was largely in the hands of AMD. Prior to the BFA Patch, the crossfire profile was working flawlessly for me (2 x 580 8gbs). In order for crossfire to work with DX11, exclusive fullscreen (EFS) needs to be enabled in game. Blizz, removed EFS in DX11 for various reasons, that benefit some and have a negative impact to others.
2.) In order to get mGPU's working while using the DX12 API, the game developer has to explicitly add the support for it. DX12 takes the control away from the hardware developers and passes it to the game developers.
My big question is - IS there anything AMD is able to do, in order to get crossfire working with a windowed fullscreen with DX11 in their wow crossfire profile?
Blizz is largely ignoring this, they appear to be sticking to their position that this was a positive change. Maybe it is, but form a players perspective there's no way to tell the complete breadth of this issue.
Here's a forum thread on the WOW forums that may shed some light on the ordeal, along with some supplemental twitter spam: