Is it possible for an ObjectDB PersistenceManger/EntityManager to connect to more than one database at a time? Also, can objects in one database reference objects in another database (e.g. through globally unique identifiers)?
Using multiple databases
You can use as many databases as needed at the same time but each database connection requires a separate EntityManager/PersistenceManager.
References to objects in another database are not supported. Of course, you can keep IDs of the other database objects and manage the references yourself. You will not be able, however, to run one query on both databases.
ObjectDB - Fast Object Database for Java (JPA/JDO)