One thing to remember is that when the air outside is already warmer than the air you want to heat or cooler than the air you want to cool, this will significan

One thing to remember is that when the air outside is already warmer than the air you want to heat or cooler than the air you want to cool, this will significantly increase the efficiency of the heat pump in achieving that temperature. I would guess to the point where it's not worth complicating the system in this way. (Especially since it will be a relatively rare compared to the reverse situation.)