What if one application (EAR) uses multiple datasources for different appliciation parts (modules, EJB jars) but only one physical database (Oracle) let's say from legacy reasons. Different entities can use different datasources in single transaction. Everything works but how much is this solution effective?

Are there any hidden costs for such configuration?
How inteligent is EntityManager to handle such case?
Aren't unnecessary 2PCs (two phase commit) triggered?