Scald crabs with hot water to stun. Remove the back (top shell) from each crab, and clean out gills, lungs, and center of each. Crack crabs in half, and remove claws. Reserve claws for stock.
In a large heavy stockpot, combine oil and flour, and heat over medium heat. Stirring slowly, make a dark-brown roux, 15 to 20 minutes. Add onion, bell peppers, and celery. Cook, stirring frequently, until vegetables are soft, about 5 minutes. Add bay leaves, salt, cayenne, and seafood stock. Stir to combine. Bring mixture to a boil over high heat; reduce heat to medium-low, add crabs, and simmer 20 minutes. Add crabmeat and shrimp, and cook 10 minutes. Remove from heat, and add parsley. Serve hot.
Recipe by Louisiana Cookin' at https://louisianacookin.com/shrimp-4-ways/