You can combine information from multiple images by using multiple "-tagsfromfile" options on the same command line. For instance:

But in the case of Canon THM can CRW images, this isn't really necessary. The CRW contains all of the useful information found in the THM image. The only additional information in the THM are EXIF-specific values which are always constant or can be derived from other information. I own a 300D myself, and I just ignore the THM files in my workflow.

For me, most of the interesting information is in the maker notes, which contains the same information in both types of images. Much of this information will be lost in the translation to XMP.