Hops added at the beginning of the boil are bittering hops simply because the 60 minute boil makes the hop oil isomerize which make them bitter. If you add the same hops at the end of the boil the oils do not become bitter, the aroma is not driven off by boiling and they are aroma hops then. Hops added in the middle of the boil become flavoring hops because part of the oils that give aroma are driven off but not all so we don't really smell them but perceive it as flavor. Use one packed at the beginning of the boil and one at the end (or when the recipe states).