UPDATE: I’ve since been told an even better way.
I script as much as possible for Rubenerd, including image processing. This is how I’ve been concatenating images side by side for the longest time:
$ montage -border 0 \ -geometry x$_ORIGINALHEIGHT \ -tile 2x \ $_IMAGE1 $_IMAGE2 \ out.ext
But there’s an easier way!
$ montage -mode concatenate $_IMAGE1 $_IMG2 $_OUT
I don’t have an install to try in GraphicsMagick, but my hunch is its ImageMagick-specific.
More code or lines in your scripts are rewarding, but not half as much as being able to take lines out.