Reduce oven temperature to 350ºF. Cut small slice off both ends of each potato. Cut potatoes crosswise in half; scoop out centres into medium bowl, leaving 1/8-inch-thick shells. Mash potato flesh. Squeeze head of garlic to remove roasted cloves. Add to potato flesh with sour cream, 1-1/4 cups cheddar and Parmesan; beat until fluffy. Spoon into shells, mounding as necessary to fit; place in shallow baking dish.