That is exactly my reasoning. I think most people who have their HVAC replaced think of it all as one unit. But it is really 2 units that run through the same distribution system, and therefore both could qualify for the credit. AC needs to be 16seer or greater and Furnace needs to have 97% or greater AFUE rating. The exception is a heat pump which is all one unit for both heat and cool outside the house. 

But I'm glad my initial question wasn't as "silly" as I thought. It sparked a good conversation. Thanks to all who contributed!