If you are talking steel, which is the only thing that can be parkerized, The best way to do coat it would be to parkerize first, then Cerakote. I have heard that the parkerize etches the surface for the cerakote to stick to.
I parkerize some of my bolt knobs, It is a very easy process. Cerokote has two types of coatings. One is baked on and the other is very slow curing (like a week).