The examples of his multiboxing seem to suggest he would just put his characters in a fresh instance/raid and then control them all at once. If he is using just windows 10 features i'd imagine the client stacking would be done with the 'virtual desktop', or the built in Snap feature (FancyZones maybe?).

With his key broadcasting? I have no clue what he is on about. He mentions playing in real time then talks about just adding a delay in the repeater.