Hi, I think the Adrenalin won't be able to handle both since they are from two different families. It should work fine if its from the same 6000 Series with 2-Way Crossfire X. Still unlike with switching between APU and GPU from Adrenalin, this way GPU Workload settings will apply to all AMD Radeon GPUs.
Besides, any software trying to handle on Direct X 12 should run instabilities because towards of it's latest features, where some API requirements are not met by the Polaris architecture. And specially with the core difference between Polaris, is that RDNA reorganizes the fundamental components of GCN for a higher IPC, lower latency, and better efficiency.
I'm not sure if you could try it with two different Windows Boot options? Though still it'll be frustrating to boot onto the other OS just for FluidMotion.