When we add a multi-medium release by means of Picard, we should:
1) add each medium as a standalone one-medium release;
2) next merge each this incomplete one-medium release to the first one.
The same is when we add a medium(s) to an existing incomplete release.
Example of editing history:
http://musicbrainz.org/release/f4379e3c-153c-4e28-8b35-33fddcdd5ad3/edits?page=1
MB Picard should be able to add a medium to a release.