Melt 1 tablespoon of the butter in large skillet over medium heat. Add onions and sugar. Cook, stirring often, 25 to 30 minutes or until onions are deep golden brown. Remove from heat; set aside.
Meanwhile, place red potatoes and enough water to cover in large Dutch oven; bring to boiling over medium-high heat. Reduce heat to medium; cook 20 minutes or until tender. Drain. Repeat procedure with sweet potatoes.
Preheat oven to 350°F. Coat 3-quart oval baking dish with nonstick cooking spray.
Beat red potatoes at medium speed with an electric mixer until mashed. Add 3/4 cup of the milk, sour cream and 2 tablespoons of the butter, beating until smooth. Stir in 1 teaspoon salt and pepper. Spoon into prepared baking dish.
Beat sweet potatoes until mashed; add remaining 3/4 cup milk and remaining 2 tablespoons butter, beating until smooth. Stir in remaining 1/2 teaspoon salt. Spread over red potato mixture; top with half of onion mixture; swirl gently with a knife. Top with remaining onion mixture.
Cover. Bake 25 to 30 minutes or until thoroughly heated.
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