Interleave

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

Interleaves frames from several clips on a frame-by-frame basis – 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.

See filters with multiple input clips for the resulting clip properties.

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

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