In a small, heavy saucepan, combine blueberries, peaches and 2 tablespoons water; cover and cook over medium-low heat until blueberries begin to burst, 3 to 5 minutes. In a cup, combine cornstarch and 2 tablespoons water; stir into blueberry mixture along with brown sugar. Cook, stirring occasionally, until thickened, 2 to 3 minutes. Spoon blueberry-peach mixture into dessert bowls or plates, dividing equally; sprinkle with granola. Serve warm with frozen yogurt or ice cream, if desired.