Dick:
One minor advantage of using two magnets with the Protege is that the zeroing at the start is more intuitive as it is done after rolling slightly forward. Another is that you do not have to add one revolution after riding for more than a halfmarathon, although hardly any measurers ever do this.
However the disadvatages far outweigh these advantages. An extra magnet can be inconvenient to obtain. Also, an inattentive measurer is twice as likely to create spurious revolutions.