Preheat oven to 400*F., with racks in center and lower third of oven. Place garlic in a small piece of foil; drizzlel with oil. Fold to form and packet, crimping edges to seal. Prick potatoes. Place garlic packet and the potatoeson center rack. Place a baking pan on the lower rack to catch juices.
Bake, turning half way through, until sweet potatoes are very tender when pierced with knife, about 45 minutes.
Remove sweet potatoes and garlic from oven; continue to bake the russet potatoes until very tender, about 15 more minutes. Remove from oven, let cool about 5 minutes.
Peel potatoes. Pass through a ricer or a food mill into a medium bowl(or mash by hand with potato masher). Squeeze garlic from skins into bowl with potatoes. Stir in butter, parsley, sage, salt, and pepper, and yogurt. Stir to mix. Serve and enjoy!