If you write both FileSource tags, you should be able to read them both back. Try it from the command line and use the -v2 option to see what is written. If both were written, you should see both when you read the file with the -a option.

The only value specified by the EXIF spec is 3 (Digital Camera), but I have found other references that use 1 (Film Scanner) and 2 (Reflection Print Scanner). But I just noticed that I added the values of 1 and 2 to the EXIF FileSource tag, but not the XMP. I will fix this.

- Phil