#!/bin/bash

mkdir out
for a in *.jpg; do
  convert "$a" -crop 50%x100% ./out/"${a%.jpg}".jpg; done

#ImageMagick renames the left half of the crop to end with 0, need the right half alphabetically first
cd out
for b in *0.jpg; do
  mv "$b" "./${b%.jpg}"2.jpg; done