I'm slightly worried you might be hitting multiple different bugs there. But instead of worrying, maybe just try the 'modesetting' driver. It might solve even more problems if you get both GPUs using the same driver:
Section "OutputClass" Identifier "Use Modesetting" MatchDriver "radeon|amdgpu" Driver "modesetting" EndSection
I'm not sure why those drivers don't yet prefer "modesetting" by default because in Wayland sessions that's exactly what happens anyway.
I'm slightly worried you might be hitting multiple different bugs there. But instead of worrying, maybe just try the 'modesetting' driver. It might solve even more problems if you get both GPUs using the same driver:
Section "OutputClass"
Identifier "Use Modesetting"
MatchDriver "radeon|amdgpu"
Driver "modesetting"
EndSection
I'm not sure why those drivers don't yet prefer "modesetting" by default because in Wayland sessions that's exactly what happens anyway.