I also have a 400k btu (input) propane heater for my pool / spa. Propane is about 91.5k btu per gallon. Running the heater for an hour uses about 4.4 gallons of propane. Propane delivered costs me $3.40 a gallon = about 15 bucks per hour to run the heater. I use the heater for the spa often — takes about a half hour in the Fall or early Spring when the water is below 70 degrees F, much less if the water is warmer. But quick to heat is also quick to cool. Keeping it warm 24/7 would be very expensive. I’ve used the heater to heat the entire pool exactly twice, for pool parties. I has been a while since I did the calculations but I recall for my 15k gallon pool, it costs me over $100 to raise the temp by 20 degrees F, one time. Every day, not a chance. Had to make it clear to my dear wife early on, ah, no, we won’t be regularly heating the pool lol.
We also have the 1,000 gallon propane tank, underground. I forget the numbers but it was much less expensive for us to get a high pressure tap off that tank and run over 100’ of gas pipe to the heater, than it would have been to obtain and have installed a second tank. Our propane co. makes a Spring delivery that runs the pool and smaller stuff all Summer, and makes a second delivery just after Christmas that then runs the heat for the house, so there’s not any risk of running out, but if the pool heater were also active in Winter, it would be a different story.
If you’re still contemplating heating the spa in Winter, a separate tank isn’t really a “fire wall” against running out because if the separate spa tank runs out, sure, the house is warm, but the spa equipment and piping might be destroyed. Either way you’d be well served to calculate* the propane needed by the spa based on historical temps. You can also use that to determine if you really need a second tank or just more frequent deliveries. The propane co. can likely help with that. I’d also note it’s easier to remember to every so often keep an eye on the propane level in one tank than two, but if you have a good memory, that might not be a factor.
The way my spa is set up (I assume most are set up similarly), it is possible to completely isolate the spa from the pool and leave the spa open in the cold but our builder advised against leaving it open if freezing is expected. Lots can go wrong that is really hard and expensive to fix.
*calculating propane use is pretty easy: 1 btu will raise 1 lb of water by 1 degree F. A gallon of water is about 8.3 lb. A gallon of propane supplies about 91.5k btu. Your heater is likely about 80% efficient in using those propane btu’s.