I have been dreaming about a split keyboard with dual trackballs and two cursors for the past few months. This may be the kick i need to actually get on it.
They didn't call them mouse cursors with Ataris and Commodores because almost nobody had heard of a mouse, but there was more than one joystick port so multi-player was built-in from the beginning.
I hadn't heard of a mouse either but when the trackball appeared, mainly to play Centipedes and Missile Command, I used it as a single-button mouse anyway, not only to point & click but to drag & drop which I had never seen demonstrated.
So I made a little 2-player game, not fancy, each player stayed in their own window.
Of course nobody had ever heard of "Windows" either since MS-DOS wasn't even out yet.
In fairness to the creator, it's a niche product that would have taken quite some time to develop.
I quite like BenVallack's video about the Svalboard [1] (and his videos in general about his journey through the world of split keyboards).
[0] https://svalboard.com/
[1] https://youtube.com/watch?v=-Lz_FNoYHNM