I always found it disappointing that desktop environments usually only support a single mouse pointer and a single focused window / insertion point for keyboard input.

Unixy systems might support multihead where devices could be routed to separate desktops, but I'm thinking more like

If you had a very large 4k tv hooked up two a computer with two keyboards and two mice/trackpads, and did pair programming with a colleague or a child, able to edit separate windows/tabs or the same one simultaneously.

like if i could have pair programmed with my dad when i was a kid? or together with my brother on the little games we made together?

that would have been amazeballs for learning and fun