Like christian said, a sealant over a sealant won't really change the look. It would be the same as 2 coats of 1 or the other. A carnuba over the sealant will give you a different look.
You can layer different sealants, like killrwheels said, and make sure the top layer is pure sealant . I don't think it's going to do a lot for you though. But go ahead and try it out.