Heat oven to 350°F. Lightly spray 13x9-inch (3-quart) baking dish with cooking spray. Cook spaghetti with 1/2 teaspoon salt according to package directions. Drain well, and place in baking dish.
In large skillet, brown sausage over medium-high heat 5 minutes, stirring occasionally, until pork is no longer pink. Drain and set aside. Wipe skillet clean. Add pepperoni, and cook over medium-high heat 4 minutes, stirring occasionally, until slightly crisp.
Top spaghetti in baking dish with sausage; pour pasta sauce over sausage. Arrange half of pepperoni slices evenly over pasta sauce. Sprinkle evenly with cheeses. Arrange remaining half of pepperoni slices evenly over cheese. Cover with nonstick or lightly greased aluminum foil.
Bake 30 minutes; remove foil, and bake 10 minutes or until cheese is melted and just begins to brown.