Pierce potatoes several times with a fork and place on a microwave-safe plate. Microwave on high for 4-1/2 minutes or until tender. Allow potatoes to cool slightly.
Meanwhile, in a bowl, combine the Alfredo sauce, garlic powder, pepper and thyme. Stir in 1/2 cup cheddar cheese and mozzarella cheese. Cut potatoes in half lengthwise. Scoop out the pulp and add to the sauce mixture; mix well. Spoon into potato shells. Sprinkle with remaining cheddar cheese. Microwave on high for 45 seconds or until cheese is melted.