Place sweet potatoes in a large pot with enough water to cover, and bring to a boil. Boil until tender, then cool, peel, and cut into 1/4 inch slices.
Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13 inch baking dish.
Arrange half the sweet potatoes in the bottom of the prepared baking dish. Layer half of the apples over the sweet potatoes. In a small bowl, mix together brown sugar, salt, and mace, then sprinkle half of the mixture over the apple layer. Dot with half the butter. Repeat layers of sweet potato and apple, and top with remaining brown sugar mixture and butter.
Bake in the preheated oven for 50 minutes, until apples are tender and top is golden brown.