I do get away with one treatment for my Carniolan hives. They do have a brood break in winter though. So by the time the autumn treatments come out of the hive there is no brood in the hives. They only start raising brood again in early spring/late winter,so there is no chance of varroa to breed in the hives. As long as most of the beekeepers around you treat their hives around the same time and there is no re invasion of varroa into the treated hives it works well.
With Italians there is always some brood in the hive through out the winter. But if the daughter queens from my breeder show