If you take the car to Powerflow they can sort out fitting the duel exhaust
you may not need to buy a whole new system if you already have a system by them, as the duel exhaust uses a 2nd back box which connect in via a split pipe just before the main backbox...
the 2nd backbox should be a smaller box, obviously with a matching tail pipe
they should also be able to cut the approriate hole in your back bumper, but will be upto you to get your hands on a 2nd heatshield.