Preheat oven to 350°. In a large skillet, heat butter over medium-high heat. Add onion; cook and stir until crisp-tender, 3-4 minutes. Add shrimp and snapper; cook until shrimp turn pink, 5-7 minutes. Remove from pan., In the same pan, cook and stir salsa, cream cheese, chiles and salt over medium heat just until cream cheese is melted. Stir in 1 cup shredded cheese; remove from heat., Spread 1 cup cheese sauce into a greased 13x9-in. baking dish. Gently stir remaining cheese sauce into shrimp mixture. Place 1/3 cup shrimp mixture off center on each tortilla. Roll up and place in prepared dish, seam side down. Sprinkle with remaining cheese., Cover and bake until heated through and cheese is melted, 20-25 minutes. Sprinkle with cilantro before serving.