I am exporting photos from Apple Photos and importing them into darktable. Apple Photos uses basename.xmp, which darktable can read. When darktable makes its own edits, though, it writes a basename.ext.xmp file, leaving Apple’s basename.xmp in place.
The above is documented in the darktable manual; what I’m left unsure about, though, is what happens if I want to migrate to some other software in the future. Will having two XMP files that correspond to the same image cause problems? Should I rename Apple’s basename.xmp to basename.ext.xmp before adding the photos to darktable, or just leave Apple’s XMP files next to darktable ones and not worry about it?