Currently it is possible to copy a rem as a tag, select multiple other rem, and paste that tag, applying it to each of the selected rem. By analogy, copying a reference and selecting multiple other rem should paste that reference, optionally as a pin, at the end of the selected rem.
The main use case is adding a source link to multiple resulting (or updated) notes in bulk, so depending how the 1.6 multiple sources turn out, there could be a separate toggle to append to the end of the sources field (less granular, but still useful).