Which OS do you use? I don't know about Windows, but I know with linux you can (with a lot of effort, not for the faint of heart) setup two monitors and two input devices, so you could play the cabinet while someone else uses the main monitor for something else.
I saw a similear gadget for Windows 95 (which that was current), but nothing since.
Beware of video cards. Matrox is known to work well in the configuration, others may or may not work well. Do your research first to make sure your combonation will work. (Most of today's high end cards will work, but not all, older cards are much less likely to work, or might only work in a certian combonation.)
Like I said, it is a hastle to set up. You need USB keybaords, and you need need to be comfortable editing configuration files directly. Basicly you set up two screens, and need to tell the computer to use keybaord 1 with screen 1...