Place sweet potatoes in a large saucepan; cover with water. Bring to a boil
Reduce heat; cover and cook for 20-25 min or just until tender.
Meanwhile, place the Yukon Gold potatoes in another large saucepan; cover with water. Bring to a boil.
Reduce heat; cover and cook for 15-20 min. or until tender.
Drain Yukon Gold potatoes; mash with ¾ c milk, half of the cream cheese and 1 Tbsp butter.
Add onions, ¾ tsp salt and pepper; mix well.
Spoon into a greased 13x9 in baking dish.
Drain sweet potatoes; mash with the thyme and remaining milk, cream cheese, butter and salt.
Spread over potato mixture.
Cut through layers with a knife to swirl.
Bake, uncovered, at 350 for 25-30 min. or until heated through.