Time for some TIPS!
Tip number 1 - when running dual monitors at 4k your frame rate caps at 30 fps.
Tip number 2 - if you are plugging a new monitor you will need to reboot as the raspberry pi does not actively search for new monitors when running.
Tip number 3 - There are many screen options to choose from to get the best results out of the Raspberry Pi OS that you can find out about in the guide.
OSs should have similar Windows INDIVIDUALISM treatment for dual monitors:
-- wallpaper
-- panel(taskbar)
-- panel apps (apps appear only on the monitor/s they occupy)
-- panel customization : position/orientation, size, auto-hide, etc.
-- geometry : resolutions, panel relative positions to each other
In short, each monitor should be treated much like a DESKTOP area