Spray 1 1/2-quart casserole dish with cooking spray; set aside.
In large mixing bowl, combine spinach, artichoke hearts, mayonnaise, sour cream, 1/2 cup Parmesan cheese, and the mozzarella cheese. Stir until combined. Add the cayenne pepper and paprika and combine well.
Place into prepared casserole dish. Bake for 30 minutes. The last 5 minutes add the remaining 1/2 cup of Parmesan to the top.