Seriously. I don't know that it can be done on Linux. There was a user on protondb that claimed the game works "even with lots of mods" but there's no way to message people on there and ask them how the fuck they got something to work....
For years I was stuck with a setup where my main monitor was plugged into the GPU via HDMI, but my secondary monitor only had VGA, for which the card did not have a port. My motherboard does have one, though. Using that basically worked... with random issues popping up both now on Linux and way back in the dark times....
Currently, if I upgrade to 555 drivers, my second monitor will not show a display despite it being detected by Plasma and xrandr. Additionally, if I do get past the log screen on wayland, my framerate/refresh rate is abysmal but not reported by xrandr. Unplugging the second monitor fixes the issue....