1. Cook spaghetti with 1/2 tsp. salt according to package directions. Drain well, and place in a lightly greased 13- x 9-inch baking dish.
2. Brown sausage in a large skillet over medium-high heat, stirring occasionally, 5 minutes or until meat crumbles and is no longer pink. Drain and set aside. Wipe skillet clean. Add pepperoni, and cook over medium-high heat, stirring occasionally, 4 minutes or until slightly crisp.
3. 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.
4. Bake at 350° for 30 minutes; remove foil, and bake 10 more minutes or until cheese is melted and just begins to brown.