Preheat oven to 350°.
Remove white papery skin from garlic heads (do not peel or separate the cloves). Wrap each head separately in foil. Bake at 350° for 1 hour; cool 10 minutes. Separate cloves; squeeze to extract garlic pulp. Discard skins. Set garlic aside.
Increase oven temperature to 400°.
Remove casings from sausage. Cook sausage in a large nonstick skillet over medium heat until browned, stirring to crumble. Remove from pan with a slotted spoon. Place sausage in a large bowl; stir in sage and rosemary.
Melt butter in a large saucepan over medium heat. Lightly spoon flour into a dry measuring cup; level with a knife. Add the flour to melted butter, stirring with a whisk. Gradually add the milk; cook until slightly thick, stirring constantly with a whisk (about 10 minutes). Stir in roasted garlic, cheeses, salt, and pepper. Remove mixture from heat. Add 5 1/2 cups cheese sauce and cooked pasta to sausage, stirring to coat. Spoon pasta mixture in a 13 x 9-inch baking dish coated with cooking spray. Top with remaining sauce. Bake at 400° for 15 minutes or until thoroughly heated.