PREPARATION:

1. Preheat oven to 450°F. Pierce potatoes with fork in several places. Bake directly on rack 45 minutes or until fork-tender. Reduce oven to 350°F.
2. Meanwhile, cook bacon in medium skillet over medium-high heat 6 to 8 minutes or until crisp. Remove from heat; transfer bacon to paper towels with slotted spoon.
3. Add onions to bacon fat in skillet and cook about 12 minutes over medium-high heat or until golden brown. Remove onions from skillet; set aside. Stir buttermilk into skillet, scraping up any browned bits from bottom of pan. Add butter, stirring until melted.
4. Cut baking potatoes in half lengthwise with serrated knife; carefully scoop out flesh into large bowl. Reserve skins. Add 3/4 of buttermilk mixture, 1/2 teaspoon salt and 3/4 cooked onions. Mash with potato masher until smooth.
5. Cut sweet potatoes lengthwise with serrated knife and scoop out flesh into medium bowl. Discard skins. Add remaining buttermilk mixture, 1/4 teaspoon salt and remaining cooked onions. Mash with potato masher until smooth.
6. Fill half of each reserved potato skin horizontally, vertically or diagonally with baked potato mixture; fill other half with sweet potato mixture. Top each stuffed potato half with half slice of bacon. Transfer stuffed potatoes to baking sheet; bake 15 minutes or until heated through.