I think the best option would be a monitor that supports picture by picture, like some dell monitors(2007wfp, 2407 wfp). You can connect 1 pc via vga or dvi and the other via s-video or RCA and split the screen. another option is a kvm switch but that only lets you switch between pc's, so not really what you're looking for. There are some KVM's that split a monitor into 4 displays, but there extremly expensive ($3000+).