In normal UNIX, a tty device pairs one keyboard with one monitor. KGI bends that rule a little bit. As we already mentioned, you can omit the keyboard or the monitor. But, there's a second way you can bend the rules.
Suppose you have two keyboards: a standard keyboard, and a braille keyboard. You load the driver for the standard one, and tell it to attach to the first 16 ttys. You then load the driver for the braille keyboard, and you also tell it to use the same 16 ttys. You've created an overlap --- two keyboards, sharing the same dev files. In these cases, KGI merges the stream of characters coming from the two keyboards. This allows the sighted and the blind user to share the machine without reconfiguring the keyboard.
This also works with mice. Suppose you like to use the mouse for your word processing, but you like to use a graphics tablet for CAD work. You can load your mouse and your tablet driver into the same "dev" files. KGI merges the streams of events from the two mice into a single comprehensible stream. If you run the X server, you can move the pointer by dragging the mouse, or by using the tablet. This allows you to use your word processor and your CAD program, without restarting the X server in between.
[Technically, I know it's still possible to tell the events from the merged items apart. I think that's too much detail for a paper of this level.]