Cheryl, why not consider putting up a shade-house connected to the existing greenhouse, rather than adding another greenhouse? I don't know about you, but my greenhouses in Mississippi were completely uninhabitable by May due to the heat. All they were good for was rooting plumeria cuttings and growing some succulents.
My shadehouse here in Vero Beach is actually a metal-framed, HF car canopy. I've never used the canopy though, since I am growing orchids in the structure. I use shade-cloth instead. You can always wrap the sides (and use the canopy) if you wanted to use the structure during the really cold months. It is 10x20'.
