Preheat oven to 375°F. Boil a large pot of salted water and cook jumbo shells until just barely al dente, about 1 to 2 minutes less than package directions. Drain and spread on a lightly oiled baking sheet to prevent sticking.

In a skillet over medium heat, melt 1 tablespoon butter and add minced garlic. Cook for 1 minute until fragrant, then add chopped shrimp. Cook just until pink, about 2 minutes. Remove from heat immediately.

In a large bowl, combine softened cream cheese, ricotta, mozzarella, lemon juice, Old Bay, salt, pepper, and parsley. Stir until mostly smooth. Gently fold in the cooked shrimp and crab meat, keeping some crab chunks intact.

Using the same skillet, melt remaining butter over medium heat. Add garlic and cook 1 minute. Pour in heavy cream and broth, simmer 3 to 4 minutes until slightly thickened. Stir in Parmesan, season with salt, pepper, and red pepper flakes. Pour half the sauce into the bottom of a 9x13 baking dish.

Spoon the seafood filling into each shell using a regular spoon. Nestle filled shells into the baking dish over the sauce. Pour remaining sauce over the top and scatter extra mozzarella over everything.

Cover with foil and bake for 20 minutes. Remove foil for the last 5 minutes to lightly brown the cheese. Let rest for 5 minutes before serving.