Pierce sweet potatoes with a fork; arrange in a circle on paper towels in microwave. Microwave at high 12 minutes or until tender, rearranging potatoes after 6 minutes. Let stand for 5 minutes. Peel potatoes; mash. Add 1/2 cup sour cream, sugar, 2 tablespoons butter, 1/2 teaspoon salt, and cinnamon, stirring until blended. Keep warm.
Heat mashed potatoes in a large microwave-safe bowl according to package directions, omitting salt and fat. Add milk, remaining 1/2 cup sour cream, remaining 2 tablespoons butter, remaining 1/2 teaspoon salt, and pepper, stirring to combine. Spoon sweet potato mixture over mashed potatoes, swirling with a spoon. (Do not blend in completely.)