Preheat oven to 350°F. Mix cream cheese, mayonnaise, sour cream, chiles, Seasoning Mix and 1/4 cup of the shredded cheese in large bowl until well blended. Stir in chicken.
Place tostada bowls on foil-lined baking sheet. Spoon dip evenly into the 2 tostada bowls. Top each bowl with remaining 1/4 cup shredded cheese.
Bake 25 minutes or until heated through. Serve with desired toppings and tortilla chips.