you can do it even with the 'cheapy ones'... i've got a kvm switch that only requires double tapping the shift bar then pressing '1' or '2' to choose your computer. it only cost about 40 bucks, but that was about 3 years ago. you can now find KVM switches that are USB and DVI, VGA and DVI, VGA and PS/2, etc. they're all comparable, and they can go from 2 computers to 4 (without becoming expensive).
any kvm switch changes the keyboard, video (monitor), and mouse (hence kvm) between computers. some have a hub you can use, and some only work between two computers. you don't need an expensive one to do what you ask. as long as you can view (and control via keyboard and mouse) the other computer's screens, you're good to go.
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