I would recommend a video editing program, of the “non-linear” kind. does the job, requires no programming.
you would drag the video clips into a timeline and adjust their positions in time.
video editing programs take care of differing frame rates and all that complicated stuff.
don’t bother trying to get that done with OpenCV. it isn’t made for that. and don’t even think of “seeking” in a video with OpenCV, that’s broken in many cases.
by the way: tagging a thread as opencv in this forum, which is called “OpenCV”, is very pointless.