No way to create second X screen assigned to second graphics card
I found a (possible) solution to the problem and it consists of setting nvidia-drm.modeset kernel parameter to '1', i.e. nvidia-drm.modeset=1, and using xrandr.
No xorg.conf is required anymore and must be deleted to not interfere with xrandr.
More on that: https://forums.developer.nvidia.com/t/manjaro-linux-trouble-creating-additional-xscreen/200434