Hi,
If it is a submit connection you need to have two connections to the same web service method and for one connection you bind to main data source and for other one you can bind it to secondary data source.
If it a receive connection you can set the webservice parameter value in the code. Basically when you want to submit the main data source you will set main data source XML to the webservice parameter (through code) and you will do it same for Secondary data connection