I agree about anything over 27' in the south needs 2 units. My 32' Avenger would reach 90 degrees in the daytime with its single 13.5k unit. I thought my new tracer had a 15k on it because the salesman told me that all of the 3175's on the lot had 15k units. It wasn't until I got it home that I found out it is only a 13.5k. I blame myself, I should have verified instead of taking his word for it.
After some thinking and since I eventually want both units to be 15k BTU, I believe I am going to leave the 13.5k on the front and buy a 15k to put over the bedroom. Then if I am still not happy I will buy another 15k to put over the living area and sell the 13.5k.
My next question, how tough is it to get the lap sealant off of the roof vent so I can install the new a/c?