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.
You can share a keyboard and mouse across multiple Windows and macOS computers, if you know the right tools to use.
How many computers do you use? I have a laptop and a fully decked out gaming rig—and it can be a pain to control them. Instead of stacking my keyboards on top of each other, there is specialized ...