Preheat oven to 500 degrees F (260 degrees C).

Combine soft bread crumbs, dry bread crumbs, and milk in a large bowl and allow to soak until the bread crumbs absorb most of the milk, about 10 minutes. Add ground beef, Italian sausage, 1 cup marinara sauce, eggs, garlic, Italian seasoning, and red pepper flakes and mix until thoroughly combined. Form into 16 large meatballs.

Heat 1 cup olive oil in a large pot or Dutch oven over medium-high heat. Cook meatballs until browned, about 10 minutes. Transfer to a baking sheet and bake in the preheated oven until no longer pink in the center, about 10 minutes.

Bring 1 cup marinara sauce to a simmer in a large saucepan over medium heat. Add cooked meatballs and heat through, about 5 minutes.

Heat 1 tablespoon olive oil in a large skillet over medium-high heat; cook and stir onion, red bell pepper, and serrano pepper in the hot oil until onion softens, 5 to 6 minutes. Add Marsala wine, salt, and black pepper; cook until wine has evaporated and onion turns golden, about 10 minutes.

Spread each hoagie bun with 1/4 cup mascarpone cheese and 1 tablespoon garlic basil spread. Place four meatballs and a spoonful of marinara sauce on top. Sprinkle each sandwich with 1 cup mozzarella cheese.

Place sandwiches onto a baking sheet and bake in the preheated oven until cheese is melted and buns are toasted, about 5 minutes. Top each sandwich with 1/4 of the onion mixture; garnish each sandwich with 2 tablespoons of basil and 2 tablespoons of Parmesan cheese.