Mix the cream cheese & sour cream in a large mixing bowl until completely blended. Add the horseradish, salt, and pepper & mix well. Stir in crabmeat, shrimp, onions, and 1/4 c cheese. Spoon into a 12" baking dish and top with remaining cheese. Drizzle with cocktail sauce. Bake at 350 for 30-40 mins, until center is hot. Serve with your favorite crackers or pull-apart bread.