FFmpeg command line for showing two videos side by side

Here is command line for playing two videos side by side in sync in FFmpeg (ffplay). It can be useful for comaring videos, for example.

ffplay -v warning -f rawvideo -s 800x400 -i /dev/zero -vf 'movie=video1.mkv,scale=400x400 [mv2] ; movie=video2.mkv,scale=400x400 [mv1]; [in][mv1] overlay=0:0 [tmp]; [tmp][mv2] overlay=400:0'

Is there some better way, in particular:

• Without using fake video input;

• More easily extendible, without inventing names for [tmp];

• Maybe with automatic geometry calculation, without manual coordinates;

• Input video filenames should be more easily visible and together?

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

StackExchange Code Review Q#117610, answer score: 7