Biztalk can't do this easily, because it can only transform one file at a time. With Mercator, you can add multiple file "cards" into your transaction and play them against each other, but BizTalk can only perform a 1 to 1 transform.
One approach you can try is to transform with pure XSLT and you can maybe add the second file as an external entity it is unclear if this is well supported; my hunch is that you won't be able to see the external file at runtime because of permission issues.
Or you can load one of the file into a database and lookup against that.
[b]Or you can use an orchestration to envelop both files into one XML file and thereby turn the data into one XML input. This is the best sounding solution to me so far.[/b]
Can any orchestration experts describe how this may be done using orchestration to pull from the two data sources and create an XML output? This would be the concept of using orchestration as mapping. I think if there are too many fields involved this may be a tedious solution.
-wa