A customer should be able to move content between Brandfolders. If I receive content from an agency via a Brandfolder, you should be able to drag it into another Brandfolder.
Our Bulk Move functionality allows you to move assets between Brandfolders within the same Organization. A few caveats apply: the user moving assets must have Collaborator access or above on both Brandfolders and a matching section type (Documents, Logos, People, etc.) must exist in both Brandfolders.
Head over to our knowledgebase for more information.