Interleave

Interleave(clip1, clip2 [, ...])

Interleave interleaves frames from several clips on a frame-by-frame basis, so for example if you give three arguments, the first three frames of the output video are the first frames of the three source clips, the next three frames are the second frames of the source clips, and so on.

The last frame of the shorter clip is repeated until the end of the longest clip. The audio is taken from the first clip.

Interleave is very useful for comparing two similar videos, stepping frame-by-frame:

Interleave(
\    A.Subtitle("A"), 
\    B.Subtitle("B"))