Same change for custom is_names columns as for the standard authors column: separate by &

src/calibre/library/catalogs/csv_xml.py

@@ -118,6 +118,9 @@ class CSV_XML(CatalogPlugin):
                     if field.startswith('#'):
                         item = db.get_field(entry['id'], field, index_is_id=True)
                         if isinstance(item, (list, tuple)):
+                            if fm.get(field, {}).get('display', {}).get('is_names', False):
+                                item = ' & '.join(item)
+                            else:
                                 item = ', '.join(item)
                     elif field == 'library_name':
                         item = current_library