I think X11 terminology may be causing some confusion. Refer to https://nouveau.freedesktop.org/MultiMonitorDesktop.html

The normal, usable way to have multiple monitors for your X11 desktop environment is for them to all be combined into one logical screen that you can move windows around in, and that applications aware of the right extensions can discover the actual physical layout of the monitors that comprise the single logical screen. Multiple screens in that X11 sense is a far more obscure feature than simply supporting more than one physical monitor.