They can join at Bromley North but at Grove Park where they have to change to another service it's stairs all the way. On very rare occasions if Bromley South is closed for works then trains will run through from Bromley North to Charing Cross or Cannon St and then PRM can dare to use this service.
As a pure terminus station without a regular through service to Central London it must be unique in Greater London. Windsor and E C and St Albans Abbey are the next nearest.