You will need to add your second Xml file as a Base64 string to the Message field of your message. This will require a C# helper class and making the message element a distinguished field
Message.message = ConvertXmlToBase64(XMlDocument secondXml);
Then use MTOM encoding on the Binding tab of the WCF Transport Properties.
You will need to confirm with the web service publisher that MTOM is the correct attchment mechanism.