Well, I'm not sure of the exhaust layout for your specific vehicle, so I'll go over some of the details of my vehicle (yeah, yeah...not really the same, but may give a better understanding.)
In my car (BMW 540) the exhaust runs from the manifolds to the cats. After the cats the two sides meet in a "silencer" and go down to a single exhaust running the rest of the length, turning around the spare wheel well (located in the bottom of the trunk).
Now, in my car, running a "true" dual exhaust would be a simple task if it weren't for the spare wheel well. For whatever reason they made it oddly shaped so there was only enough space on one side for the exhaust to pass by. The M5 on the other hand with true dual exhaust has a spare wheel well that is shaped just fine for both sides to go by.
Basically, that was all wasted typing because all you really need to know is your clearance on both sides of where the exhaust would be routed. Assuming nothing is in the way all you would really need to do is find a general muffler shop (preferably one that does mandrel bends, though this would make things a bit pricier) pick yourself up some magnaflow mufflers (or whatever you prefer) and have them route a system. Simple and effective. Especially because you are just routing from the cats which would lend you a "true dual."