The vo has to be convinced of your case, folks usually just provide offer letter (shows your role that will be matching first noc), promotion letter or reference letter ( that shows your roles/responsibilities that will be matching second noc ). When they can't find above info in these letters, then they look for additional documentation like reference letters. So depending upon what info is included in the experience letter that you hold, you might need separate reference letters for both or one letter for second role only.