In medium saucepan, melt butter over medium heat. Add garlic and sauté 1 minute.
Whisk in flour; cook 1 minute, whisking constantly. Gradually add in warmed half & half and milk, whisking constantly until smooth. Continue cooking, whisking constantly, until thickened and mixture comes to a boil.
Blend in 1½ cups of each cheese; salt and pepper to taste. Reduce heat to low and continue stirring until cheese is melted.
Place thawed potato rounds in a grease 13x9-inch baking dish. Pour cheese sauce over potatoes. Sprinkle with remaining cheese.
Bake in a preheated 350°F oven 40-45 minutes or until bubbly and golden brown. Remove from oven and let stand 10 minutes before serving.