Place potatoes in a large pot and cover with water. Bring to a boil then reduce to a simmer and cook until potatoes are tender. Drain well. Add sour cream, cream cheese, milk, garlic powder, salt, and pepper. Mash until creamy. Stir in 1 1/2 cups cheddar, Parmesan cheese, and 1/2 of the bacon.
Transfer to a lightly greased 9x13 baking pan. Sprinkle with remaining cheese and bacon. Bake uncovered at 350 degrees F for 25-30 minutes or until heated through and cheese is melted. Top with green onions before serving.