For those interested in something similar with sway, you can:

- Add a headless display with `swaymsg create_output SOME_NAME`

- Configure its resolution and position like you do for other outputs

- Start a VNC server on that output with `wayvnc --output=SOME_NAME`

- Start a VNC client somewhere else you want to use your second screen (there are some that are browser-based)

Unfortunately it is somewhat undocumented: https://github.com/swaywm/sway/issues/5553 but you can find some info on the net: https://www.reddit.com/r/swaywm/comments/k1zl41/thank_you_de...

I saw in that readme that they wanted ways to get rid of dummy adapters. That above is one such possible way for sway.