Preheat oven to 350°F. Melt butter in a small skillet over medium-high. Add celery, onion, and garlic; cook, stirring often, until vegetables begin to soften, 6 to 8 minutes.

Stir together celery mixture, salt, Italian seasoning, pepper, eggs, 1/3 cup of the ketchup, and 1 tablespoon of the Worcestershire sauce in a large bowl until combined. Add turkey, Italian sausage, breadcrumbs, and Parmesan cheese; mix gently with hands or a large fork until just combined. Shape mixture into a 10- x 5-inch loaf on a parchment paper-lined rimmed baking sheet.

Bake in preheated oven 35 minutes.Meanwhile, stir together tomato sauce, tomato paste, and remaining 1 tablespoon each Worcestershire sauce and ketchup. After meatloaf bakes for 35 minutes, remove from oven. Spoon tomato mixture over meatloaf, and return to oven. Bake at 350°F until sauce is glazed and a thermometer inserted in center registers 165°F, 10 to 15 minutes. Let stand 10 minutes before slicing.