Cook pasta according to package. Meanwhile, in a small skillet, saute green pepper and onion in 1 teaspoon butter until tender and set aside.
In a large bowl, combine the crab, shrimp, egg, mozzarella cheese, mayonnaise, 2 tablespoons milk, 1 teaspoon seafood seasoning, pepper and green pepper mixture.
Drain and rinse pasta then stuff each shell with 1 rounded tablespoon of seafood mixture. Place in a greased 13x9 baking dish.
In a small saucepan, melt remaining butter over medium heat. Whisk in flour and coarsely ground pepper; gradually whisk in remaining milk. Bring to a boil and cook and stir for 2 minutes or until thickened. Stir in Parmesan cheese.
Pour over stuffed shells. Sprinkle with remaining seafood seasoning. Bake uncovered at 350° for 30-35 minutes.