Conversation ID and procesDocument

Here is the scenario. Please provide help.

1.) I am doing RosettaNet PIP 3A4 Responder.

2.) I have 2 IS Servers. one for PIP mapping and one for backend Data Logic and DB insert.

3.) the backend IS does not have access to any TN.

4.) I have to use a remote invoke to send the BackEnd Document to backend IS.

5.) I will create the conversation ID from a service I will write and pass the conversation ID with the backend document and the backend will maintain the Conversation ID in the system with request Document.

6.) for PO Response, backend IS will invoke processDocument via Remote Invoke on my IS to send the document to my TN.
(backend will call the processDocument with coneversationID came in Request document.)

Questions.

I am not using processDocument for request instead, I am using remote invoke for self written service on backend IS for sending document to backEnd IS, how will response join the conversation when I never send the request document to my TN and where should I put the Conversation ID so when processDocument is called for the resposne it would match it to the open conversation.

should I call the processDocument for request and on completing it call the service on the backend via remote Invoke and pass the same ConversationID that processDocument used? (sounds dirty to me)

PROS!!! Please provide possible solutions I will really appreciate that.