Thank you for writing back. When people have an over flow spa in Florida I like to have at least 100,000 BTU's. Then you don't have to wait forever for the water to heat up. Since you have two separate bodies of water then you either need two heaters or there is a way to pipe it with (3) 3 way valves but it is involved. Some people will use a cheap $550.00 11 KW electric element heater on the spa and a heat pump on the pool. Some people with the spa as a separate body of water get a very small heat pump and then just keep the water warm all the time and use a fiberglass cover to keep the heat in. Then you will use very little power to maintain the temperature as the heat up time is not a factor. Hayward and AquaCal make quality heaters, Unfortunately they both just cut their warranties in Florida from 5 years parts and labor to 2 years parts and labor. This seems to be a trend in Florida right now. Not the right direction if you ask me though.
If you have any questions please give me a call and I will be more than happy to help you.