I have a B550 + 3950X.
RX5700XT in the main gpu slot
GT1030 via the chipset
Installing with the dual GPUs was a /pain/, but once you get it installed  it seems to work mostly.
Networking works (but you need to set sys-firewall to not auto start)
Qubes will use the GT1030 with nouveau drivers, but struggling to get hardware acceleration working (currently driving 16m pixels feels noticeably laggy)
The RX5700XT seems to pass through to a VM without issue (needed to tweak the vm kernel args to remove nomodeset, but then the amdgpu kernel module loads and I get the expected device nodes) but I haven’t figured out how to persuade the vm to actually use the GPU for anything (I suspect I just need to RTFM on that front)
I have not had any issues with IOMMU or similar. (Though I did go through the bios with a fine toothed comb to enable all the IOMMU and similar things)
 I ended up patching the installer to include kernel-latest but that seems unnecessary. What seems required is forcing the installer to use the fbdev. Either kill anaconda in a second screen and start the install with anaconda —usefbx or (untested) add inst.usefbx
 I needed to explicitly set the Driver and BusID in the xconfig otherwise it gets confused by the AMD GPU and refuses to launch X.