When assigning a Tag to a raw picture, DigiKam creates an sidecar file which includes the raw file ending, e.g. D80_1234.nef.xmp If there is a corresponding jpg another (D80_1234.jpg.xmp) sidecar is produced when metadata for this is edited. If there is a "generic" sidecar file (e.g. D80_1234.xmp) digiKam uses this one as combined xmp for the raw and the jpg. I propose a feature: Setting for "Create single xmp for raw and jpg", which would change the behaviour in all functions which create / update .xmp features to not include the file-type in the .xmp file name. This would require to automatically update the metadata shown of all pictures related to this xmp if this is changed. Example: For D80_1234.jpg the star rating is changed. This is written to D80_1234.xmp, which would trigger that the star rating shown for D80_1234.nef would also need to be updated (and accordingly for other metadata).
Even if we strongly advise against the sidecar scheme BASENAME.xmp, as it can lead to possible problems, this possibility already exists. In digiKam Settings-> Metadata-> Sidecar tab-> Compatible filenames for commercial programs. Maik