Place potatoes in a 3-qt slow-cooker. Arrange smashed garlic on top. Cook potatoes on high 3-4 hours (or on low 5-6 hours) until potatoes are very tender.
Using an electric handheld mixer (or alternatively a potato masher) beat potatoes until partially mashed. Add cheeses, butter, milk, and salt and pepper to taste to potatoes and beat on medium speed until potatoes are smooth and creamy. Keep potatoes on the *warm* setting in the crockpot until ready to serve.