It goes from each manifold into the two cats, one being on each side as you've noticed. Then runs down into that "muffler" looking object, that is a resonator. From there, the factory only uses one pipe, which leads to your muffle which sits side-way by your gas tank. From there it goes to the rear "muffler" you see behind the car, that, is also a resonator.
Expensive system to replace with factory parts!
You can easily convert it to duals, just cut off the first resonator and run two pipes back. That'll give you a true dual exhaust which will give you an excellent boost in performance and sound!
If your on a budget or not after spending a lot and dealing with the work, just replace your main muffler (one sitting side ways) with a one-in-two-out muffler. Then run two pipes to the tips, where ever you like. Deleting any and all resonators is completely optional.