FileFluss

It doesn’t exactly combine multiple cloud storage services, but it’s close.

Ideally, the application itself would handle distributing the files across different services and maintain an index of where they are hosted, somewhat like a torrent. While it doesn’t have this feature for automation, you can certainly do it manually.

You can add several of your accounts and manage the files stored in them from a single place, and it even allows you to move files between them.