As a Galaxy admin, you can configure Galaxy to distribute data on multiple persistence media (e.g., your local storage, or Amazon S3) using object_store_config.xml. One application of this setting is to use multiple storage media when a single media is not big enough to persist all your datasets (e.g., use multiple hard disks on a computer). However, this configuration will set Galaxy to distribute datasets regardless of which user has created them.
At the moment, you cannot configure Galaxy to distribute datasets on a per user-based setting (what you're asking). We are working on adding this functionality to Galaxy, you can follow the development at PR #4840.