Using two heaters

I currently have a small 10 x 20 kidney inground pool (10k gallons) with a spillover spa (800 gallons). I live in a hot dry climate and use a Coates 18KW electric heater. The cost is inexpensive due to the abundance of hydroelectric power, however in the winter I want to have the spa only running while the pool is winterized. I can do this with the valve settings and keep the water from spilling over by lowering the water level and circulating only the spa water. The problem is that during the winter, temps will be 10 to 30 degrees. In order to keep the spa running I will keep the electric heater running at 55-60 but want to add a propane heater that will bring the temperature up quickly. I currently have a propane stub out that can accommodate up to 125k BTU. I have two questions:

1. Does anyone have recommendations on propane heater, I am thinking a Pentair Minimax 120k BTU above ground heater,

2. Does the propane heater go before or after the electric heater.

My goal is to only use the propane heater to bring the temperature up quickly. The spa is only 800 gallons, so I think 120k BTU should be fine. I am mostly unclear whether it should go before or after the current Coates heater.

Welcome to the forum.
Can't make any recommendation on a heater because I don't know details of that heater. However, I would put the gas heater ahead of the electric. With the electric heateer you get 100% of the energy converted to heat because the heat source, the resistance heater, is in the circulating water. With a gas heater you want the maximum temperature differential on the heat exchanger to get maximum efficiency. If you had the gas after the electric you would be pre-heating the water and reducing this differential and thus the efficiency. Probably not much of a difference either way but every little bit of efficiency gain helps. If anyone sees anything wrong with this thinking please speak up.
Al