Split screen (video production)

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. There may or may not be an explicit borderline. Until the arrival of digital technology, a split screen in films 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. (Wikipedia)