most easy way would probably be:
he should upload those image again/twice, and use one time the first image, and one time the second image.
a other way would be:
work with a repeater and image field instead of gallery field
a other way would be:
use a repeater field with a gallery inside and each time he wish to reuse a image he need to create a new gallery. and you create a single slideshow out of those gallerys at/for frontend. (probably a little more complicate loop than now)
or:
create your own gallery-field plugin. i think it would need at least 2 fields one that contains the image-pool, and one that contains the slideshow. also a upload functionality and a way to remove add and ordering the images inside the slideshow
