I'm going to install a Modine Hot Dawg 45K heater in my detached garage. In the garage I have a pool heater (200K) that is vented through the roof. Can I just connect the garage heater exhaust to the same stack (double wall pipe) as the pool heater?
Not if that stack is going to be too small for the additional 45K BTUs. You need to get a gas fitter to check the code book for proper chimney sizing. A permit should be taken and the unit properly inspected. Fail to do so and your insurance may be void if there is a fire.