1 muffler w/ 2 pipes is not true dual exhaust. It just looks like it. Is your mustang a V8 car or ?
If it's a V8 car, I would take it to a shop and have a true dual system put on. If it's not, then yes, you can do what your brother has done and get a muffler with dual tips.
Look at JC Whitney, Jegs, Summit, or your local auto parts store and somebody should be able to hook you up.