In X Windows, a “Display” can actually refer to a collection of monitors. Every display has a number of screens. The full form of an X Windows display address is “hostname:display.screen”
(This is why “export DISPLAY=:0” refers to the default local display)
It sounds like what you want is for an application to run on multiple screens attached to the same display, as if it was an individual screen.
You can do this by arranging your screens using
--pos for where each monitor “lives.”
Then just resize the window to cover the full range.
However, this is approximately what the monitors control panel does, so why doesn’t this work as-is for you? What behavior are you seeing that doesn’t work right?
If you want to create your own multi-screen setup, you can use
xrandr to combine them. Something like:
xrandr --setmonitor MyMonitorName auto DP-0,HDMI-1
(The names have to match your configuration, of course)