Heat oven to 350°F. In 2-quart saucepan, melt butter over medium-high heat. Cook onion and garlic in butter 3 to 4 minutes, stirring frequently, until softened. Stir in flour until all flour is blended with butter. Stir in broth and sour cream until smooth. Heat to boiling; cook about 2 minutes, stirring constantly, until sauce is thickened. Remove from heat. Stir in cheese until melted. Stir in chiles.
In medium bowl, mix 1 cup of cheese sauce, the crabmeat and shrimp. In 13x9-inch (3-quart) glass baking dish, spoon 1 cup of the cheese sauce over bottom. Spoon about 1/2 cup seafood mixture onto each tortilla. Roll up tortillas; place seam sides down on sauce in dish. Pour remaining sauce over enchiladas.
Cover baking dish with foil. Bake 30 to 35 minutes or until bubbly.
In medium bowl, toss remaining ingredients. Just before serving, spoon avocado mixture down center of enchiladas.