I've seen tons of cars with dual exhausts and they all have one common issue. If the pipe routing is done like the one in that picture, only one tip actually works. When the car is running, it's kind of funny to see exhaust only coming out one tip. And after a little while, one tip is really dirty while the other one is pristine. Also, sometimes the muffler shop hacks the job so bad the car probably lost power. I've seen it where the welds at the joint are really bad and block flow. Other than that, it really doesn't do anything.