Actually, increasing the volume of a vehicle's exhaust discharge reduces back pressure, therefore improving the engine's overall effiency and performance.
Exhaust that is forced through too small of an opening has resistance, which makes the engine work harder, loose power, heat up faster, develop cabon deposits, wear out sooner and even shortens the lofe of every exhaust component and your oxygen sensors.
In the long run, dual exhaust is in many ways very beneficial
It definitely won't increase gas consumption so don't worry, I'm not sure how it is set up but I'm guessing the dual exhaust is just for looks although it may decrease back pressure in your exhaust system which can up HP. So don't be worried it's mostly for show.