I'm not a Mac user but something like this would be nice for me. I'm visually impaired, and I have to be very close to the screen so I have my main monitor on an arm so it can float above my keyboard. Then I have a second screen (or laptop) on a desk next to me, but to see that I have to turn 90 degrees and lean in, which makes it awkward to use my keyboard/mouse. This is better with a laptop because it gives me a second keyboard/mouse for the second monitor, but I have to keep moving the cursor back and fourth. My solution so far has been to use Autohotkey to define some hotkeys that move my mouse to the center of my main display and the center of my secondary display, which works surprisingly well.

Although to be honest, I rarely use the second display. I mainly use it while screen sharing in a meeting so I can have notes and stuff up on my main display.