Bring 6 quarts water to a boil in a large saucepan. Add 1 tablespoon salt and pasta; cook 6 minutes or until al dente. Drain.
Place flour in a medium saucepan over medium heat; add 1/2 cup milk, stirring with a whisk until smooth. Gradually add remaining 1 1/2 cups milk to pan, stirring with a whisk; bring to a boil, stirring constantly with a whisk. Cook 2 minutes or until slightly thick, stirring constantly; stir in butter. Remove from heat; let stand 4 minutes or until sauce cools to 155°. Add cheeses; stir until smooth. Stir in remaining 1/2 teaspoon salt, pepper, and pasta. Garnish with parsley, if desired.