I purchased a 120v walll heater for my bathroom. I also purchased the optional wall mounted double pole thermostat from the same vendor. I have wired many home projects but I never worked on a double pole device. The power supply I want to tap into also feeds the motor to the bathroom hot tub. The tub motor is rated at 120v 20 amp. With what appears to be a 10 gauge romex power source and is dedicated at the circuit breaker. The heater is factory wired to draw a maximum 1500w and 12.5A. With one power line how do I wire the thermostat and wall heater (heater and thermostat both have two red and two black wires and a ground)?

I would run a new circuit for the heater cause the hot tub probably is on a gfi if you try to break of it.. it would probably wind up tripping it..

A) You did not need or want a double-pole thermostat. You need a single-pole.

B) As 14u2c states, DO NOT use the feed to the tub. This most likely is required to be a dedicated circuit and tapping into it would 1) be a violation, and 2) not be smart because you would NOT be able to use both items at the same time.

Run a new circuit.

Can't he use the double pole thermostat, and parallel the contacts to reduce the load of one set, or just use 1 set of poles?

He sure could. I would just use one set as opposed to paralleling them.
I guess the cost difference between the two would not make sense to go exchange it. :o

The thing to remember with many two pole thermostats, is one pole is only a switch, closes as soon as the stat is turned from "OFF" to a higher temp setting. The other pole has the actual thermal sensing action, will close when temp falls.

If only the temperature sensing pole is used, it is never a positive open when OFF is needed, can close anytime as temp falls.

A single pole stat is needed when used for a 120 volt device.