Can multiple versions of virtual edits be stored in the same .XMP sidecar file, or is a separate .XMP sidecar required for each different set of virtual edits? I know one can create "virtual copies" in Lightroom but, as far as I can tell, Lightroom stores all but the first "copy's" editing information within Lightroom's database and not in the .XMP sidecar file. I also know that DarkTable creates a separate XMP sidecar file for each version of edits. But DarkTable's file naming scheme for these .XMP sidecar files is unfortunate and unique to DarkTable.
Unfortunately you are right, XMP file store the changes of original file. Edits of virtual copies are stored in to the LR catalog. Do not be mislead by the naming of files on export, they are related only for this case and not the internal information.
I personally do not use cataloging features of LR and found no way to store my edited virtual copies.