FFmpeg command line for showing two videos side by side
Partial result

ffmpeg -v warning -i video1.mkv -i video2.mkv -filter_complex '[0:v]scale=400:400,pad=800:400 [0:q]; [1:v]scale=400:400[1:q]; [0:q][1:q]overlay=400:0' -f nut -c:v rawvideo -c:a copy - | mplayer -noconsolecontrols -cache-min 1 -cache 1024000 -



Dummy video input removed
Filenames now separate command line aguments, not parts of a longer string
Still manual geometry
Still not automatically extendible to more videos
Preparing and playing video decoupled, now can use other player instead of mplayer (unfortunately mpv or vlc fails to provide seeking in this scenario)
Audio is preserved