How to split an image vertically using the command line

Try something like this:

montage file1.jpg file2.jpg -geometry +0+0 -background none output.jpg

This will make the border between images as small as possible and whatever is there will be transparent.

To see a demo of the difference using builtin images, try these and compare:

$ montage rose: -resize 100x60 rose: -geometry +0+0 -background none montage.jpg
$ display montage.jpg &
$ montage rose: -resize 100x60 rose: montage.jpg
$ display montage.jpg &

See Montage Usage.

If you post an example of what you're getting and manually edit together an example of what you'd like as a result, we might be able to get a little closer to that.

Here are examples that I like better than the ones I originally posted above:

montage \( rose: -resize 100x46\! \) rose: -background gray montage.jpg

montage \( rose: -resize 100x46\! \) rose: -geometry +0+0 -background none montage.jpg

I think you can get away with using split's --filter=COMMAND.

... | split -b <SIZE> -d - part --filter=./split-filter

where ./split-filter is something like

#!/bin/bash

set -e

n="${FILE#part}"
case $((10#$n%3)) in
    0)
        dd bs=64K >"path1/$FILE"
        ;;
    1)
        dd bs=64K >"path2/$FILE"
        ;;
    2)
        dd bs=64K >"path3/$FILE"
        ;;
esac