Juan, Thanks for the links and references. What I was saying is the fact that he has a Hibernate Session that will be wrapped by an Entity Manager. As you mention, all the entity managers will share the same session and that's why he will need to inject his current session inside an entity manager. If it's the same session and he is not using another transactional resource there will be just one transaction to commit or rollback so there will be no need to distributed transactions configurations.
Cheers