I would imagine you could go to a local muffler shop and see if they have the ability to bend 4" pipe. My friend did that on his and he paid $200. Out of curiosity why do you want to go duals? From what i understand they only add to the cost of the system and give you no more power/mpg than any single aftermarket system. When considering aluminum coated pipe, keep in mind diesel exhaust is oil based which will make the pipes last longer. In that case, Farmboys makes a great kit and I have a tip I will sell for $50, 4" in and 5" out chrome, still in original plastic
good luck!