Vista - duplex channel client-side behaviour

Hi all,
i have a simple contract with an associated callback contract (over a simple
NetTcpBinding). I instantiated two clients. When the clients are called back
by the service i notice the callbacks being executed on different threads
than the one where both clients were constructed. Further i can observe that
callbacks for the same client are always on the same thread. Can I rely on
this thread-affine callback-behaviour for each each client or could it be a
simple coincdence? Is there a (hidden) session involved (due to using duplex
pattern)?
Thanks in advance
Peter