Creating a side-by-side video from two input files requires knowledge of the input file dimensions, mainly to extend the overlay to a proper size and applying the overlay. Most often, this is manually specified in the command line, but with some constraints some of it can be automated in a makefile.
If the left-most video dimensions are larger than the right, the following makefile will place the videos side-by-side and pull the audio from the left-most file.
$ cat Makefile
all: sideBySide.mp4
left.mp4: ../big_buck_bunny_1080p_h264.mov
${SH} ffmpeg -i $< -filter:v "crop=1024:768:0:0" -ss 60 -t 10 $@
right.mp4: ../big_buck_bunny_1080p_h264.mov
${SH} ffmpeg -i $< -filter:v "crop=1280:720:0:0" -t 15 $@
%.size: %
${SH} ffprobe -v error -show_entries stream=width,height $< > $@
#--generate side-by-side, assumes left image is larger (or equally large) to right image
sideBySide.mp4: left.mp4 right.mp4 left.mp4.info right.mp4.info
${SH} export left=$(shell echo $^ | cut -d ' ' -f 1); \
export right=$(shell echo $^ | cut -d ' ' -f 2); \
export W1=$(shell grep "^width=" left.mp4.info | cut -f 2 -d '='); \
export H1=$(shell grep "^height=" left.mp4.info | cut -f 2 -d '='); \
export W2=$(shell grep "^width=" right.mp4.info | cut -f 2 -d '='); \
export H2=$(shell grep "^height=" right.mp4.info | cut -f 2 -d '='); \
export Y=$(shell echo '\($$H1-$$H2\)/2'); \
ffmpeg -i $$left -i $$right -filter_complex "[0:v]pad=$$W1+$$W2:0:color=gray[int];\
[int][1:v]overlay=$$W1:0+$$Y[vid]" -map "[vid]" -map 0:a -acodec copy $@
%.info: %
${SH} ffprobe -v error -show_format -show_streams $< > $@
clean:
${RM} *.mp4 *.info
