A program or script to overlay several video clips I have a few hundred few-second-long video clips (c001.mp4-c999.mp4)
and need to merge them into a clip slideshow.  This is very easy - I drop them into iMovie and click "slideshow", done! 

However - I want to fit more clips in less time.  So I was hoping to show 2 clips at once, and stack vertical-video clips side-by-side.

Ideal would be some way to take a script of start times and upper-left positions, and render it to a combined flattened video?  (assuming basics like "no resizing" and "later clips are stacked on top of earlier clips if the earlier clip hasn't finished playing yet")

file:c001.mp4, t:0:05:15, x:500, y:0 
file:c002.mp4, t:0:09:25, x:0, y:0
file:c003.mp4, t:0:12:00, x:500, y:0
etc.
While having some fun in Python programming language I found a library that allows users to write custom code to overlay (possibly) infinite amount of clips.

MoviePy



Hope this helps!