simplest with vstack (2)

Watch on youtube.com

doc

https://ffmpeg.org/ffmpeg-filters.html#vstack, https://ffmpeg.org/ffmpeg-filters.html#amerge, https://ffmpeg.org/ffmpeg-filters.html#pan

This is also a simple approach, maintaining the aspect ratio. Just personally I do not like this because each video size is too small.

#! /bin/sh
pref="`basename $0 .sh`"
vleft="EuropeanArchive.mp4"
vright="Eduardo.mp4"

#
ffmpeg -y -i "${vleft}" -i "${vright}" -filter_complex "
[0:v]scale=960:540,pad=1920:540:(ow-iw)/2:0,setsar=1[0v];
[1:v]scale=960:540,pad=1920:540:(ow-iw)/2:0,setsar=1[1v];
[0v][1v]vstack[v];

[0:a][1:a]amerge=inputs=2,pan=stereo|c0<c0+c1|c1<c2+c3[a]
" -map '[v]' -map '[a]' \
  "${pref}.mp4"