Shell and devein the shrimp, wash them and refrigerate until ready to use. In a 12-quart pot, bring the water, chilies, lemon slices, bay leaves, thyme and salt to a boil. Drop in the fresh crabs and boil briskly, uncovered, for 5 minutes. Remove the crabs from the pot and set aside to cool. Drop the shrimp into the stock remaining in the pot and cook 3-5 minutes, or until firm to the touch. Remove the shrimp to a plate and cool. Boil the stock, uncovered, until it is reduced to 3 quarts. Strain the stock through a sieve and discard the seasonings. Cover the pot to keep the stock warm.
When the crabs are cool enough to handle, shell and reserve the meat, leaving the large claws intact. In a 6-quart pot, warm the roux over low heat, stirring constantly. Add the onions and garlic and stir for 5 minutes. Add the okra and green peppers and mix well. Stirring constantly, pour in the reserved warm stock in a slow, thin stream and bring to a boil over high heat. Add the cayenne pepper, Tabasco and season to taste with salt and pepper; simmer for 30 minutes. Stir in the crabmeat and claws and simmer, over low heat, for an additional 30 minutes. Add the shrimp and simmer 5 minutes longer, then taste for seasoning. Serve at once with rice. 4 servings.