Preheat broiler. Halve and pit the peach and nectarine. Line a shallow baking dish with foil; coat foil with cooking spray. Place fruit cut side down in prepared dish.
Broil 6 in. from the heat for 3 minutes; turn fruit over and sprinkle with brown sugar. Broil until sugar is melted and bubbly, 2-3 minutes longer. Serve warm with whipped cream or ice cream if desired.