SessionNotFound error during an active session

Hi,
I am new to this and I am doing something very simple, I think.

2 separate threads in the same class. In the 1st thread messages can be sent & received without any problem.
In the 2nd thread it throws a SessionNotFound error when calling sendToTarget();
The session is active, as my counterpart at the other end of the session confirmed.
Same class; same session; 2 separate threads.

I tried defining a separate initiator in the 2nd thread. That got a Config error

Thanks in advance for any help,
Robert