One of the pipes is the supply to the engine, the other is the return pipe, which brings the excess fuel from the injectors back to the tank. You can’t tap into the return pipe, as it will not go far enough down into the tank. I wouldn’t tap into the fuel supply as there’s a good chance of exhausting the fuel in the tank, meaning you couldn’t run the engine. Best solution is to put a separate off take into the tank for the heater, making sure it doesn’t go all the way to the bottom of the tank, as Yngmar as already said.