CARAMELIZE THE ONIONS: Melt 1 tbsp butter in large skillet over med-high heat. Add onions and sugar and cook, stirring often, 25 minutes till onions are a golden brown and caramelized. Remove from heat, set aside.
BOIL POTATOES SEPARATELY: Meanwhile, while onions are caramelizing, place potatoes in two pots large enough for water to cover potatoes. Add the garlic cloves to the pot with the red potatoes. Bring both pots to a boil, reduce to med heat and cook until tender, about 20 minutes, then drain in colanders.
PREP RED POTATOES: Beat red potatoes and garlic with electric mixer till mashed. Add 1/2 cup milk, sour cream and 2 tbsp butter. Continue beating till smooth. Season with salt and pepper to taste.
PREP SWEET POTATOES: Beat sweet potatoes until mashed. Add remaining 3/4 cup milk and 2 tbsp butter, the brown sugar and vanilla. Beat till smooth, season with salt to taste.
ASSEMBLE: Preheat oven to 350°F Lightly grease a 3-quart baking dish. Place the red mashed potatoes in the bottom. Spread sweet potato mixture over red potato mixture. Swirl by cutting through mixture gently with the flat side of a knife. Top with the onion mixture. Cover and bake for 30 minutes. Sprinkle with bacon crumbs and serve.