Claire,
Other way you can do is instead of directly routing those xml messages to the TN gateway service (tn:receive),create some custom service which will parse the inbound document and next step will be extracting the TN ExternalIDs (create some UserDefined id’s in the partner profile)using the TN huilt-in services and include these ID’s in the XML message if you have any field holder and route it to TN for recognizing the Sender/Receiver/DocType based on the TNDocumentType/Extract Query.
But i am not sure this solution is feasible for your implementation unless you agree with this type of custom processing.
And i belive by using just with the Partner certificates TN will not able to recognize the sender/receiver.
HTH,
#webMethods#B2B-Integration#Integration-Server-and-ESB