In a large skillet, melt 2 tablespoons of butter over medium heat. Once melted, add 3 minced garlic cloves and sauté for about 1 minute until fragrant. Stir in 1 cup of lobster and 1 cup of crab meat, cooking for an additional 2-3 minutes until warmed through. Remove from heat and set aside to cool.

In a mixing bowl, combine the sautéed seafood mixture with 8 ounces of cream cheese and 1 cup of shredded mozzarella. Season with salt and pepper to taste, then mix thoroughly until the filling is well blended and creamy.

Bring a pot of salted water to a rolling boil, then add 12 jumbo pasta shells. Cook the shells for about 9-11 minutes, or until al dente. Using a slotted spoon, remove the shells and transfer them to a colander to drain, then allow them to cool gently on a clean kitchen towel.

For the sauce, melt 4 tablespoons of butter in a medium saucepan over medium heat. Whisk in 1/4 cup of flour, cooking until it turns golden, about 2 minutes. Gradually pour in 2 cups of heavy cream, whisking continuously until the mixture is smooth and thickened, approximately 3-4 minutes. Stir in 1 cup of shredded mozzarella and 1/2 cup of grated Parmesan until melted and creamy.

Preheat your oven to 375°F (190°C) while you assemble your dish. Spread a thin layer of the prepared cheese sauce on the bottom of a 9x13 inch baking dish. Stuff each cooled pasta shell generously with the seafood filling and arrange them in the dish. Pour the remaining cheese sauce evenly over the stuffed shells, ensuring they are well covered.

Cover the baking dish with aluminum foil and place it in the preheated oven. Bake for 20 minutes. After that, uncover the dish and bake for an additional 10 minutes, until the sauce is bubbling and lightly golden on top.

Once baked, remove the dish from the oven and let it rest for a few minutes. Garnish with extra Parmesan cheese or freshly chopped herbs if desired. Serve warm to enjoy the rich flavors and creamy texture.