it's much more reasonable and trivial on lower quality monitors but if one of the two PCs is for gaming you're going to want 4k at at least 120Hz which, last I checked, didn't exist (or was very expensive). You also might have a hard time finding one that takes DisplayPort in, which is preferable for Linux.

Personally I just run the USB devices into a $5 USB A/B switch and manually change the inputs on the monitor.