4 lbs Russet or Yukon Gold potatoes peeled & cubes, 3 lbs sweet potatoes peeled & cubed, 2 carton (8 oz. each) spreadable chive & onion cream cheese, 1 cup (8 oz.) sour cream, 1/4 cup shredded Colby-Monterey Jack cheese, 1/3 cup milk, 1/4 cup shredded Parmesan cheese, 1/2 t. salt, 1/2 t. pepper.
Topping: 1 cup (4 oz.) shredded Colby-Monterey Jack cheese, 1/2 cup chopped green onions, 1/4 cup shredded Parmesan cheese.
Cook Russet potatoes in Dutch oven - over with water and bring to boil; reduce heat, cover and cook 15 - 20 minutes. Place sweet potatoes in large saucepan; cover with water. Bring to boil, reduce heat; cover and cook 15 - 20 minutes. Drain, mash with half cream cheese and sour cream and all Colby-Monterey Jack cheese. Drain Russets, mash with remaining cream cheese & sour cream. Add milk, Parmesan cheese, salt, pepper, mix well.
Spread 2 2/3 cups Russet mixture into each of 2 greased 11 x 17 x 2" baking dishes. Layer with 4 cups sweet potato mixture. Repeat layers. Spread with remaining Russet mixture. Bake uncovered @ 350 degrees 15 minutes or until heated. Combine topping ingredients, sprinkle over casserole. Bake 2 - 3 minutes longer our until cheese is melted. Yields 2 casseroles (10 servings)