HEAT oven to 350ºF.
MASH potatoes in large bowl. Add sugars and cinnamon; mix well. Stir in next 4 ingredients.
LINE bottom and sides of 13x9-inch pan with pie crusts, overlapping edges in center. Fill with potato mixture.
BAKE 40 min. or until knife inserted in center comes out clean. Top with marshmallows; bake 10 min. or until lightly browned.