If you don't mind spending 600-700 dollars, Flowmaster makes a kit that's got dual exhaust and everything from the cat back... I think it's called the American Thunder kit. It sounds REALLY nice and comes with everything you'd need.
Magnaflow also makes a dual exhaust system from the cat back for around 600 to 700. It sounds very very good too. I've heard it on a 2012 Tradesmen and I fell in love with the sound. Those exhaust systems are expensive and supposedly give performance gains, but I'm not for sure on that.
It would be alot easier and cheaper to go buy exhaust tubing, with a single in dual out muffler. And I wouldn't worry about not have factory dual cut-outs on the back bumper. I love the look of a truck with a dual exhaust and no cut outs on the bumper, especially on 4th generation Rams.
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