Both cards are usable with modesetting X driver. No need to disable anything. Maybe i could try X configuration to get xf86-video-amdgpu and xf86-video-ati to load correctly. Correct solution would be to fix choosing xf86-video-amdgpu and xf86-video-ati X drivers, but i have a solution that works for me. It is not very common configuration and this problem is not a high priority for me.