There's no formal definition for any of the extended rounds: 50@50; etc, presumably Fred Rogerson and his contemparies didn't consider that it would happen with any regularity. There have been a few membership applications using extended rounds, generally handled under rule 11 of the Club's rules.

As far as a double round goes, it can be done howsoever the individual decides though if it is to be used for club membership application then it should include a standard sub 24hr round.