They used that a whole bunch for the mirror scenes in Quantum Leap where Sam would look in the mirror and see himself as whoever he leaped into and there was someone else in the scene. That series was a boon for actors who had identical twins.

The other identical twin thing I remember was an episode of Buffy the Vampire Slayer where Xander was split into two, one of him all his strengths and the other all his weaknesses. Nicholas Brendon's twin brother was used in the scenes where there were two Xanders in one shot.