Ich weiß nicht wirklich, wie die das machen, aber hier haben wohl vier Jungs mit einem Controller und gleichzeitig Supermario 1, 2, 2 (Japan) und 3 in einem Speedrun durchgezockt. Aus dem Comments auf Metafilter:
How can that be one controller?
There's a ton of reading you can do on the tasvideos website to learn about how people do this, but long story short, different games do wildly different things on a per-frame basis that let you pull of these kinds of shenanigans when you can issue input at super-human levels. For example, game 1 might only poll the controller state on even-numbered frames, and game 2 might only actually start your character moving if the controller has held down the right arrow for at least 2 frames. You send RLRLRLRLRLRLRLRLRLRLRL on the controller, game 1 sees RRRR and you run full-speed, game 2 sees RLRLRL and makes you stand still.
TAS Videos also do weird things like pressing L and R at the same time (or even UDLR), which makes most games behave in unexpected ways (e.g. game 1 might treat this as R and game 2 might treat it as "do nothing").