Oh, and I played a bit with multiseat yesterday. If you want, every VNC user can have their own mouse/keyboard, with independent focus.
Though that currently takes a bit of fiddling (`swaymsg seat seat_name attach "virtual pointer"` + one wayvnc instance per pointer), wayvnc reportedly has a branch where it's done automatically.
- 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.