Best practice is to use the filters independently. The more glass you put in front of the lens the more you degrade the image quality.
Though you *might* be able to stack the CP on top of the UV depending on how thick they are. At the wide end of the lens you may get some cropping though. You can find out pretty easily by putting the CP on with the UV and taking a bright daylight shot at 18mm. If you see shadow around the edges of the pic then you have your definitive answer.
