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.)
Multiple mice is esoteric.
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.
If you have a USB mouse then you have multiple mice too, in that sense.
Multiple mice (1 pointer) is not esoteric at all... I have a completely normal scenario with 3 (normal mouse, track pad, track point) right now.
And at home, I have a mouse on the left side of the keyboard and another one on the right.