It's been asked a few times about using the stock mufflers on a true dual setup on the harleys.Stock mufflers for the most part will not work on a true dual setup.
The key is in the construction of the muffler. Some mufflers you can look inside the shell front and rear and see if they are both constructed the same as far as the baffling like the older ones were but the newer ones are totally different between the right and the left mufflers. The left is basically used as a sound chamber to make the exhaust sound better without much provisions for flow and as such has a very short baffle in the rear of the muffler.
If you compare some slip ons from say a dyna, those would be pretty much the same muffler internally but when it comes to a bagger, the flow is desired to exit mainly from the right muffler for better scavenging of the rear cylinder and very little flow of exhaust in comparison from the left muffler.
The key is in the construction of the muffler. Some mufflers you can look inside the shell front and rear and see if they are both constructed the same as far as the baffling like the older ones were but the newer ones are totally different between the right and the left mufflers. The left is basically used as a sound chamber to make the exhaust sound better without much provisions for flow and as such has a very short baffle in the rear of the muffler.
If you compare some slip ons from say a dyna, those would be pretty much the same muffler internally but when it comes to a bagger, the flow is desired to exit mainly from the right muffler for better scavenging of the rear cylinder and very little flow of exhaust in comparison from the left muffler.