Combine lettuce, tomato, mango, red onion, red bell pepper, and cilantro in a medium bowl and mix thoroughly. Add 2 tablespoons mayonnaise, 1 teaspoon sriracha sauce, and 3/4 teaspoon lemon juice; mix together. Add salt and pepper. Refrigerate salsa until ready to serve.
Mix 2 tablespoons mayonnaise, 1 tablespoon sriracha sauce, and remaining lemon juice in a small bowl.
Coat shrimp and mahi mahi fillets with 1 1/2 tablespoons olive oil and cover with blackening and Cajun seasoning. Heat 1 tablespoon oil in a skillet over medium-high heat. Add fillets; cook until fish is golden, firm, and easily flaked with a fork, 3 to 4 minutes per side. Place fillets on a plate. Add shrimp to the pan; cook until opaque, about 3 minutes per side.
Spread a layer of the sriracha-mayonnaise sauce over each tortilla. Add shrimp to 1 tortilla and mahi mahi to the other. Add mango salsa to both.
