Heat salted water in a saucepan to boiling (use enough water to cover potatoes). Add potatoes and boil 30 minutes. Meanwhile, slice and core apples. Heat oven to 350. Drain, peel, and slice sweet potatoes. Arrange apples and sweet potatoes in an ungreased 1 1/2 quart casserole. In a small saucepan stir together cornstarch, salt, pumpkin pie spice, and apple juice. Cook, stirring constantly, until mixture thickens and boils. Boil and stir 1 minute. Pour over sweet potatoes and apples. Cover. Bake 1 hour.