Blend taco mix into sour cream and refrigerate for at least an hour or up to several hours. On an oversized platter layer the avocado on the bottom, bringing it almost to the platter rim. Top with the sour cream mixture, making a smaller ring. Sprinkle on the Monterey Jack cheese, then the cheddar cheese. Next layer the olives, followed by the tomatoes and top with the scallions. Serve with chips to scoop.