Why do some assume this has anything to do with simultaneous exhibitions? The original story was of two games going on in separate rooms.
Where did you see anything about separate rooms in the OP? If the two players were in separate rooms, they couldn't see what the simul-giver did on the other board, and the scheme would not work.
I might be mistaken but the way i recall it works is that the other players are supposed to make a move when the master returns to his/her board. Capablanca could skip making a move on one board and return to another board, and having that person be ahead in moves.
Then they would try to "pass", but I'd tell the guy that I'm not leaving the board until he moves, if I was Capa.