A multi-cloud backup is not a Chest
Files kept in more than one cloud are still separate private copies. They are not a shared Chest.
Putting a photo in two clouds makes two copies. Each copy stays in the account that holds it. That arrangement does not invite a grandparent, and it does not give wedding guests one place to add photos.
A Chest is the shared place. People join it with an invite and a display name. There is no password. OurChest does not connect those clouds in the first release.
A later independent copy, in Google Drive, Dropbox, or OneDrive, would still be a separate file. It is not part of the first release. Deleting one copy would not delete the other.