ModeShape should have a connector that allows the content in one repository (in the same engine) to be federated into a second repository. This really would be more similar to the LocalDocumentStore that uses the other repository's Infinispan cache. Events would also need to be propagated from the other repository to the federating repository. Theoretically, transactions would "just work" in that if both caches are used in a write transaction, then both would be enlisted in a given transaction.
(Note that federating content from one workspace into another in the same repository can be better accomplished via linking.)