A 2-1 system performs as a "header" system with a "collector". If you look at a drag racing car like a Pro stock car you will notice that the motorcycle exhaust system are routed together into a common larger tube called the collector. Each of these tubes are the same length which is determined by the type of engine. The diameter of the tube is optimized for flow and proper back pressure. As exhaust flow enters the collector, the pulses create a scavenging effect, creating a slight pressure drop which helps extract the following exhaust pulse in order. The benefit from this is reduced back pressure and reversion. Reversion is where sound waves travel back up the outside wall of the motorcycle exhaust pipe and during the point where both the intake and exhaust valves are both open (overlap), the reverse "pulse" can reverse flow in the intake tract. This reversion reduces the intake charge lessening the power produced. Typically in the midrange and top end is where you get the most significant gains. Off idle is improved but not as dramatically. Models like the FLH motorcycle series with dual exhausts are a compromise of form over function. The system looks good but that left hand motorcycle exhaust can actually hurt performance as it can increase reversion. If you want to get the most improvement, a good 2-1 motorcycle exhaust pipe systems over both 2-2 and slip on mufflers.