Entity Manager and Persistence Context

Hello,
While going through the JPA Specs (Page 130) , I came across the statement below.

If there is a different persistence context already associated with the JTA transaction, the container throws the EJBException.

If this is the case and I am working with two different Entity Managers pointing to two different Databases , will such operation fail?
Consider a use case in which a session bean method is interacting with two databases (2 phase commit). In this case I would have two EntityManager references. As per the statement above doing so could fail so I am a bit confused about how to achieve this?

Hope I am putting my question correctly as I myself is confused after reading this statement.

Or does the spec say or trying to say - different persistence context but for same database.

Thanks,
Amit