Heat oven to 350°F.
Lightly grease 3 quart casserole or 13x9 inch baking dish.
In large saucepan, combine l 1/3 cups of the milk, 1/4 cup margarine, 2 tablespoons onion, 1/4 teaspoon salt, 1/8 teaspoon pepper and garlic.
Bring to a rolling boil over medium high heat.
Remove from heat.
Stir in remaining 1 1/3 cups milk.
Add mashed potato flakes, stirring with fork until potatoes are of desired consistency.
Set aside.
In large bowl, combine all sweet potato ingredients; mash with potato masher.
Spoon both mixtures into greased casserole; carefully swirl mashed potatoes and mashed sweet potatoes together. (If desired, cover and refrigerate until 1 hour before serving.)
Bake at 350°F. for 45 to 60 minutes or until thoroughly heated.