StackHorizontal(clip clip1, clip clip2 [, ...])
StackVertical(clip clip1, clip clip2 [, ...])
StackHorizontal takes two or more video clips and displays them together in left-to-right order. The heights of the images and their color formats must be the same. Most other information (sound track, frame rate, etc) is taken from the first clip - see here for the resulting clip properties.
StackVertical is similar, working top-to-bottom.