BaseMosaic on Geforce type cards is limited to 5 monitors on linux, more only supported with Quadros.
Please check this thread:
https://forums.developer.nvidia.com/t/manjaro-linux-trouble-creating-additional-xscreen/200434/8
Basically:
- delete xorg.conf
- set kernel parameter nvidia-drm.modeset=1
- check two nvidia providers are avalable
- run xrandr --setprovideroutputsource NVIDIA-G0 NVIDIA-0 && xrandr --auto