Create splitscreen video using ffmpeg
Use

ffmpeg -i shorter.mp4 -i longer.mp4 -filter_complex
 "[0]trim=0:3.5,drawbox=c=black[delay];[delay][0]concat[left];
  [left][1]hstack[v]"
 -map "[v]" -map 1:a -c:a copy out.mp4


where 3.5 should be replaced with the delayed start time (in seconds) of the shorter video.