It is a shame that this doesn't actually exist. For several years now I've wished there was a way to hook 2 sets of keyboards and mice to a computer. Each would have access to their own active window set and context (as well as the other inputs active window).

For example this could be useful when utilizing certain "extreme programming" techniques where a pair of programmers work as a team at one workstation. These workstations can have 3+ monitors, so there would be plenty of room to share the desktop space.

Edit: Obviously this would have to be supported on the OS level.