A correctly mounted dual exhaust system will allow an engine to breathe better, thus improving its gas mileage. A proper dual exhaust system will have both pipes individually routed from the engine to separate mufflers.
Dual exhaust systems work as an upgrade when they help contribute to the release of emissions form the engine. A muffler that is designed to look like a dual exhaust system does not help improve the vehicles performance, as it feeds off the existing single pipe system. Also, if the duel exhaust system shares one pipe until the middle of the vehicle, the engine will not see any performance gains. These types of dual exhaust systems are installed for aesthetic reasons.
Real dual exhaust systems have two separate pipes that begin from the engine and end at two separate mufflers. Some of which, may have a smaller pipe that connects them, to help balance the back pressure of the system. Each pipe is built like a stand-alone exhaust system and allows a vehicle to double the amount of exhaust gases that leave the engine. Not only does the dual exhaust increase fuel economy, but it also gives the vehicle a deeper more aggressive sound and increases the available horsepower of the engine.