It's actually more important that each transaction center around a JTA transaction, and any resource (e.g., Session) associated with a transaction be managed and kept around for the duration of the transaction. Also, a Session that is affiliated with an active transaction cannot be affiliated with another transaction.

So technically it is possible to have multiple Session instances all participating in a single transaction, but care has to be taken that the Sessions do not attempt to make incompatible changes. Additionally, a single Session can be used in one transaction, changes saved, and the transaction committed before the same Session can be associated/used in a different transaction.

Does this help?