Is there an advantage to smashing two rocks with one swing?

The obvious advantage is time - one hit takes less time than two. And since every weapon can be found, made or recharged infinitely many times the only reason you'd ever need to worry about such durability loss is if you don't have any replacement weapons of similar strength.

The durability used is the same amount as if you hit the ore deposits one at a time. As already answered, the only advantage is the time it takes to hit the deposits.