You can use two Windows machines at once with a single keyboard and mouse without any special hardware. I had an extra laptop lying around that now connects with the same keyboard and mouse as my PC.
My partner and I have our own Windows desktop PC systems and desks but my partner's one is used very infrequently. I'd like to get rid of one desk and have both computers use one monitor, keyboard and ...
After decades of working with computers, I started to develop some wrist and arm pain a few year ago. So I switched from using whatever random mouse I could find for under $20 to using input devices ...