Preheat oven to 350 degrees. Spray a 9X13 inch baking dish with nonstick cooking spray; set aside.
In a large pot over medium-high heat, add the beef and sausage and sprinkle with salt and pepper. Brown mixture, crumbling into large pieces. Transfer to a paper towel-lined plate. Discard all but 2 tablespoons of drippings from pot. If you don't have that amount remaining, add olive oil to make up the difference.
Add the onion, garlic, and 3 teaspoons Italian seasoning and saute, over medium-high heat, until soft, about 5-7 minutes. Return beef and sausage to the pot. Add half of the marinara sauce (2 cups or 1 jar), water, and heavy cream to the onion and meat, turn the heat to low and heat through; set aside.
Meanwhile, bring another large pot of water, with 1 tablespoon of salt, to a boil. Cook ziti according to the package instructions. Drain well, then return pasta to original pot off the heat, and add the remaining half (2 cups or 1 jar) of marinara sauce to pasta and combine; set aside.
In a medium bowl, stir together ricotta, egg, remaining 1 teaspoon Italian seasoning, and nutmeg; set aside.
Spoon a thin layer, about 1 cup of meat sauce in the bottom of a 9X13 inch baking dish. Pour half the ziti in the baking dish. Spread all the ricotta mixture over ziti, then sprinkle half the parmesan, half the mozzarella, and half the sauce.
Spread the remaining ziti, then the remaining sauce, mozzarella, and Parmesan. Bake in preheated oven, uncovered 25-30 minutes or until cheese is golden and sauce is bubbly.