Pat fish fillets dry and put in a bowl. Stir together lime juice, malagueta peppers, 1 tablespoon garlic, and 1 1/2 teaspoons salt, then pour over fish and toss to combine. Marinate, covered and chilled, 30 minutes. Add shrimp and chill at least 30 minutes but no longer than 1 1/2 hours more.
Put tomatoes in bottom of a wide 5- to 6-quart heavy pot. Top with onion and remaining tablespoon garlic, then sprinkle with bell peppers. Place plantains on top of vegetables. Sprinkle evenly with remaining 1/2 teaspoon salt. Arrange fish in 1 layer on top of plantain. Sprinkle cilantro and parsley over fish, then arrange shrimp in 1 layer over herbs, reserving marinade. Pour oil and marinade evenly over mixture in pot.
Bring to a simmer, then cover pot. Adjust heat to gently simmer until vegetables are softened and have released liquid and fish is just cooked through, about 20 minutes.