How is a composite database different from an ESB in that both provide a single point of access for multiple data sources
Consider data virtualization solutions like Composite to be one level below the ESB in an enterprise architecture.  A virtual view in Composite can bridge n data sources transparently to the code in any given service.  So while you COULD implement all that multi-source joining at the ESB level, it would be much more complex, probably slower, and ultimately tightly couple your services to your data stores.  DV provides an abstraction layer so that you properly separate concerns in your architecture and allow your ESB to encapsulate business logic without getting into the dirty details of data storage locations.