The tech people answered this question at Gatecon. It's done with computers. Having an actor play a twin on TV is pretty basic. Back before computers were used, the action the "twins" did was rather simple. You might go back to 1970's shows where the trick is used and the twins never touch. In fact, they probably stood on opposite sides of the screen! If film was being used, the film was carefully exposed two different times to create the illusion. That's why you have the two twins left and right on the screen.Originally posted by Sioku
how did they get 2 Jacks on the same screen in 'tin man'? Cos that was woooonderful!