Preheat the oven to 350°F (175°C).
Cook the jumbo pasta shells in a large pot of boiling salted water according to package directions until al dente; drain in a colander and spread shells on a large plate to cool so they do not stick together.
Meanwhile, brown the ground beef and pork sausage in a large skillet over medium heat until no pink remains; drain and discard excess grease.
Transfer the cooked meat to a large mixing bowl and add the chopped bell pepper, chopped onion, grated 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.
Spread a thin layer of the remaining 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 upright in the prepared pan.
Spoon the remaining marinara sauce evenly over the filled shells, top with chopped pepperoni and the shredded mozzarella.
Bake in the preheated oven for 25–30 minutes, or until the cheese is melted and bubbly.