Cook Pasta: Boil elbow macaroni in salted water for 8 minutes until tender. Drain and set aside.
Cook Shrimp: Heat olive oil in a skillet over medium-high heat. Season shrimp with 1 tablespoon Cajun seasoning, salt, and pepper. Cook 2-3 minutes per side until pink. Remove and set aside.
Prepare Crab: Remove crab meat from shells and break into pieces. Set aside.
Make Sauce: In a saucepan, warm whole milk and evaporated milk over low heat. Whisk eggs in a bowl. Add a small amount of warm milk to eggs, whisking constantly. Pour egg mixture back into saucepan.
Add Cheese: Gradually add all four cheeses to the milk mixture, stirring until melted and smooth. Season with remaining Cajun seasoning, basil, paprika, cayenne, salt, and pepper.
Combine: Preheat oven to 350°F (175°C). Mix pasta with cheese sauce. Fold in shrimp and crab meat.
Bake: Pour into a greased 9×13 inch baking dish. Bake 25-30 minutes until golden and bubbling.