Another approach could be that (of course depending on your process) you create a session and register notification handler A then start the process that executes the notification node then goes to a wait state lets say receive task, so you could dispose the session.
After a while you load the session and register the second (B) notification handler and signal the process to continue its operation.
That should work since you register the work item handlers on the session upon its creation (either a new one or after loading it from data store).
This is the flexibility that jBPM/Drools gives you but final solution is up to you. Just my two cents ;)