Fill a large pot with water, add a generous pinch of salt (about 1 tablespoon per gallon), and bring to a rolling boil.
Add ziti and cook for 2 minutes less than package directions (about 7-8 minutes). Drain in a colander and set aside without rinsing.
Preheat the oven to 375°F (190°C) with the rack in the center position.
Heat olive oil in a large skillet or Dutch oven over medium-high heat. Add sausage and ground beef, breaking apart with a spoon. Cook for 6-8 minutes until browned, then transfer meat to a plate, leaving 1 tablespoon of fat in the pan.
Reduce heat to medium, add diced onion and sauté for 4-5 minutes until soft. Add garlic and cook for 1 minute until fragrant. Stir in tomato paste and cook for 1-2 minutes.
Pour in crushed tomatoes and marinara sauce, scraping up browned bits. Return browned meat to the pan. Season with sugar, Italian seasoning, dried basil, red pepper flakes, salt, and pepper. Simmer for 10-15 minutes until thickened.
In a large mixing bowl, combine ricotta, egg, 1/4 cup Parmesan, fresh parsley, and fresh basil. Season with salt and pepper and stir until smooth and creamy.
Add drained ziti to the ricotta bowl with about two-thirds of the meat sauce and 1 cup of shredded mozzarella. Fold together gently until evenly coated with creamy pockets of ricotta.
Grease the 9x13-inch baking dish and transfer the pasta mixture, spreading evenly. Top with remaining meat sauce, then sprinkle remaining 2 cups of mozzarella and extra Parmesan over the top.
Cover tightly with tented foil and bake for 20 minutes. Remove foil and bake uncovered for 15-20 more minutes until golden-brown and bubbling at the edges.
Remove from oven and rest for 10 minutes before serving. Garnish with fresh basil or parsley and extra Parmesan.