In the northern part of the country, double hive bodies is the standard. Where the winters are milder there is no reason that a single hive body can't be used.Afaik beekeepers here do the same thing as everywhere else: Two deeps for the hive, a queen separator and one to two supers for the honey.
No clue why it is two hive boxes and not one or three. - Because in Nature, there's ginormous bee-hives in hollow tree-trunks, far bigger than two hive-boxes… (?)
I believe it is Olympic Wilderness Apiaries in Alaska that uses two medium deeps for their hive bodies.