Preheat the oven to 350°F (175°C).
Cook the jumbo pasta shells in a large pot of salted boiling water according to package directions until al dente; drain and spread on a large dish to cool so they don’t stick.
In a large skillet over medium heat, brown the ground beef and pork sausage, breaking up meat as it cooks; drain excess grease.
Transfer the cooked meat to a large mixing bowl. Add the chopped bell pepper, chopped onion, Parmesan, dried oregano, minced garlic, sliced olives, and 1/2 cup of the marinara sauce. Season with salt and freshly ground black pepper and stir to combine.
Spoon a thin layer of marinara sauce over the bottom of a 9x13-inch baking pan.
Fill each cooled jumbo shell with a heaping spoonful of the meat mixture and arrange the filled shells in the prepared baking pan in a single layer.
Spoon the remaining marinara sauce evenly over the filled shells. Sprinkle the chopped pepperoni over the shells, then top with shredded mozzarella.
Bake in the preheated oven for 25–30 minutes, until the cheese is melted and bubbly.