Multiplexing sessions possible?

Hi
Is there a simple way to multiplex or aggregate sip sessions onto a 
single session?
for example
Client1 Client2
    |          |
concentrator
        |
   Receiver

That is, Client 1 and Client 2 have sessions with the concentrator. The 
concentrator has 1 session with the receiver and forwards messages from 
Client 1 and Client 2 to the Receiver. Forwarding data is easy, but what 
about forwarding data back to the clients (sent by the receiver)? Is 
there a way to recover a dialog by knowing the call ID?

The way im doing it now is:
1. Concentrator sets up a single session with the receiver
2. Concentrator then waits for invites from clients
3. When a client sends an invite with the concentrator, a session is 
established between the concentrator and client; the concentrator does 
not have any interaction with the receiver in terms of invites from 
clients.
4. When the session between a client and concentrator is established, 
the client begins to send data. The concentrator then forwards this data 
to the receiver. I store the call-ID in a callInfo header in the message 
sent to the receiver. I also store the call-ID and the associated 
serverTransaction in a lookup table in the concentrator
5. the Receiver sends a message back to the concentrator, with the 
call-ID in the callInfo header.
6. The concentrator looks up the serverTransaction (based on the 
call-ID), and calls serverTransaction.getDialog to recover the call id 
and local(from) and remote(to) tags.

I know my way is quite ugly and clunky, but is there a more elegant way 
of doing it?

Thanks
Andrew