In a large pot over medium-high heat, brown both sausages, breaking into small pieces, until deeply caramelized, about 10 minutes. Remove sausage and set aside.
In the same pot, sauté onion until softened, about 5 minutes. Add garlic and cook 1 minute. Return sausage to pot and add crushed tomatoes, tomato paste, basil, oregano, sugar, salt, and red pepper flakes. Simmer for 20 minutes.
Preheat oven to 375°F. In a bowl, mix ricotta, eggs, parsley, 1 cup mozzarella, and 1/2 cup Parmesan until combined.
Spread 2 cups meat sauce in a 9x13 inch baking dish. Layer 4 noodles, half the ricotta mixture, 2 cups sauce, and 1 1/2 cups mozzarella. Repeat layers once.
Top with remaining 4 noodles, remaining sauce, remaining mozzarella, and remaining Parmesan.
Cover tightly with foil and bake for 50 minutes. Remove foil and bake 15 minutes more until cheese is golden and bubbly.
Let stand for 15 minutes before cutting and serving.