The project is to heat my spa with a Raypak 266K BTU propane heater by attaching 2 40# propane cylinders to it. I need a suggestion / recommendation for a regulator. I heard that I need a 13" wc regulator. No brand or model was recommended. Thank you.

There are only a couple of regulator companies, Fisher and Marshall comes to mind.

80# of propane will produce about 6.5 hours of burn at 266,000 BTU. Not enough capacity in my opinion.
Propane characteristics referenced in this post:

Characteristic	Value
Pounds per gallon of Liquid @ 60 degF	4.23
Gallons per pound of Liquid @ 60 degF	0.236
Formula	C3H8
Specific Gravity of Gas (air=1)	1.52
Specific Gravity of Liquid @ 60 F (Water=1)	0.504
BTU per Gallon of Gas @ 60 F	91,502
BTU per lb. of Gas	21,548
BTU per Cubic Foot of Gas @ 60 F	2,488
Cubic Feet of Gas per Gallon of Liquid	36.38
Gallons per Therm (1 Therm = 100,000 BTU)	1.1
Gallons per MCF of Natural Gas (1 MCF = 1,000,000 BTU)	11

The output of an LPG tank is relative to the surface area of the liquid in the tank. Only so much propane can evaporate from a specific surface area. A 40# tank will ONLY output about 40.1 k btu's of propane per hour... at 60 degrees ambient air temperature (less output at lower temps). Hooking two tanks up in PARALLEL will only give you 80.2 k BTU's per hour. NO WHERE near enough to supply a 266K BTU heater. ANY regulator you try to use should freeze up solid in just a few minutes.