In a skillet over medium heat, add 2 teaspoons of the olive oil, onion, garlic and ground beef and pork. Season with salt and pepper. Brown the meat and the onion, about 5 minutes. Remove from the heat, and cool to mix with cheese filling.
Cook the manicotti in a large pot of heavily salted boiling water for 6 minutes. Drain and gently toss with a little olive oil to keep from sticking and allow to cool.
Combine ricotta cheese, 2 cups mozzarella cheese, 1/2 cup Parmesan, parsley and nutmeg. Mix well and stir the cooled meat mixture into the cheese mixture. Set aside.
Preheat the oven to 350°F.
Drizzle the remaining 2 teaspoons of oil in a 13x9 baking dish to coat. Spoon 1 1/2 cups of the spaghetti (marinara) sauce over the bottom of the dish.
Either place all of the meat/cheese mixture in a Ziploc bag and cut off one corner to squeeze filling into shells, OR carefully spoon filling in each manicotti by hand.
Arrange in a single layer in the prepared dish and spoon the remaining sauce over. Sprinkle the remaining 1 1/2 cups of mozzarella cheese and remaining 1/2 cup of Parmesan over the stuffed pasta. Divide the butter pieces evenly over top.
Bake the manicotti, uncovered, for about 30 to 35 minutes. Let the manicotti stand 5 minutes before serving.