Playing 2-player games on the same keyboard, now less common but it was once very popular.

Edit: huh, quite a few gamers here :)

Doing stenography is one example - at any time you could be pressing up to 2 keys with every digit on your hands. If you were creating your own steno device, it'd be easier to make it HID compatible, since every system of importance supports it and would avoid the need to write new drivers. The Plover software (http://plover.stenoknight.com/) can be used to provide steno for regular HID devices - works with any N-key roll-over keyboard.

Classic hot seat action games where 2 players share the same terminal such as Star control 2 hypermelee.

Imagine two people playing a game.

Emacs user? :-)