the use of a Split Screen

''In film and video production, split screen is the visible division of the screen, traditionally in half, but also in several simultaneous images, rupturing the illusion that the screen's frame is a seamless view of reality, similar to that of the human eye.''
Facts on the use of the split screen -

• There may or may not be an explicit borderline. Until the arrival of digital technology  in the early 1990s, a split screen was accomplished by using an optical printer to combine two or more actions filmed separately by copying them onto the same negative, called the composite.
• The arrival of digital video technology has made dividing the screen much easier to accomplish, and recent digital films and music videos have explored this possibility in depth. Sometimes the technique is used to show actions occurring simultaneously; Timecode (2000), by Mike Figgis, is a recent example where the combination is of four real time digital video cameras shown continuously for the duration of the film. The extensive use of split-screen as part of the narrative structure of a film, as in The Boston Strangler.