I don’t yet have this working, but can suggest a direction.
From AUR, I have successfully installed Deskreen. This allows mirroring. AUR (en) - virtscreen
I saw that Virtscreen might support the second screen. There’s a dependency on python-quamash that currently isn’t working. I’ve reported that at AUR (en) - python-quamash .
I’ve seen some posts where the virtual screen might be configured manually … but I would prefer a friendlier interface.