JPA Multiple Transaction Managers

Question

I have one applicationContext.xml file, and it has two org.springframework.orm.jpa.JpaTransactionManager (each with its own persistence unit, different databases) configured

Answer 1

The only situation in which you can have two Spring transaction managers is if you never have both transactions open at one time. This is not intrinsically to do with distributed transactions - the same restrictions apply even if you want the two datasources to have completely separate (but potentially overlapping in time) transaction lifecyles.

Internally Spring's transaction managers all use Spring's TransactionSynchronizationManager which keeps a bunch of critical state in static ThreadLocal variables, so transaction managers are guaranteed to stomp all over each other's state.