Multiple keyboards is trivial - unless you meant multiple active cursors. (If you've used a USB keyboard attahced to your laptop - you've already experienced multiple keyboards. Both work.)
It's not obvious if your core statement is "multiple keyboards is trivial".
Assuming that multiple mice obviously means multiple cursors while in the same comment assuming that multiple keyboards obviously does not imply multiple input cursors is in fact pretty confusing to me
Of course you're correct re: laptop and aux kb - I was thinking of "seat" (and multiple seats) being: "Wayland seat refers to one 'seat' at which a user sits and operates the computer."[0]
So a keyboard being attached to a seat, whose input might be associated w one active window, while another kb is associated with a different seat and different active window. Perhaps I'm just overthinking it.
It's not a direct answer, but Nucleus Coop (Windows only) allows playing multiple instances of the same videogame with different savefiles and allow using multiple keyboards and mice (is it mice for the hardware thing?)
It's an incredible hacky piece of software that does work by putting together like 10 oss. Incredible and cool.
Multiple mice is esoteric.