Cook pasta according to package directions; drain and rinse in cold water. Cool to room temperature.
In a large bowl, combine the cream cheese, mayonnaise, sugar, lemon juice, salt, pepper and cayenne. Gently stir in the crab, shrimp and onions. Stuff shells, about 2 tablespoons in each. Cover and refrigerate for at least 1 hour. Yield: about 3 dozen.