How to split images using ImageMagick

This script does the job :

#!/bin/bash
# $1 : first gif
# $2 : second gif

mkdir first
cd first
convert $1 x%04d.gif
cd ..
mkdir second
cd second
convert $2 x%04d.gif
cd ..

for filename in first/*
do
  filename=`basename $filename`
  montage -tile 2x1 -geometry 512x512 first/$filename second/$filename concat$filename
done
convert concat* output.gif

rm -rf first
rm -rf second
rm concat*

This should work with all the image types that ImageMagick can handle without having to specify *.png, *.jpg, *.jpeg etc:

#!/bin/bash

images=$(identify -format '%f\n' * 2>/dev/null)

IFS=$'\n'
set -e

max_dims=$(
  identify -format '%w %h\n' $images 2>/dev/null |
  awk '($1>w){w=$1} ($2>h){h=$2} END{print w"x"h}'
  )

orig_dir=originals_$(date +%Y-%m-%d_%T)
mkdir "$orig_dir"
mv -- $images "$orig_dir"
cd "$orig_dir"

set +e

for image in $images; do
  convert -- "$image" -gravity Center -extent "$max_dims" "../$image"
done

This will move the original images into a dated directory in case the results are not desirable. Also, this will fail if, for whatever reason, the image files have newlines in their names.

This script could do with some more error messages to give a helpful indication if anything went wrong. But for now if there is any error moving the images (everything between the set -e and set +e), the script will exit. Hopefully this will avoid doing any irreversible damage.

Update

Now with awk script shamelessly plagiarised from @terdon's answer.