The CLI Docker Save command allows one to save multiple images, from multiple repositories to the same stream / file. e.g.:

$ docker save centos:7 alpine:3.7 > ~/image.tar

This functionality is first-class on the API (i.e. isn't implemented by the client itself).

The currently implemented SaveImageCmd does not allow that. You can choose between all tags of a single repository or a single tag of a single repository.

SaveImageCmd could allow the same freedom of the CLI command.