Preheat oven to 400 degrees F. Grease 9-inch pan.
Combine flour, cornmeal, sugar, baking powder, and salt. Stir in milk, oil and eggs. Mix until dry ingredients are just moist. Pour into prepared pan. Bake 25 minutes or until golden brown. Cool cornbread, then break up.
Saute onion and bell pepper in oil until wilted.
Mix shrimp, crab meat, onion and bell pepper mixture, salt and pepper to taste with cornbread. Use some of the reserved boiled shrimp stock to moisten.
Place in 2-quart casserole dish. Place 1/2 stick of butter, sliced, on top of mixture.
Bake at 450 degrees F until golden brown, approximately 5-10 minutes.