Mix both the flours together with some water. Add salt to taste. Cover and keep it in a warm place for about 7-8 hrs to ferment. Heat a flat pan (Thava) and Grease it with a spoon of oil. Add some water in the dosa batter and make it into a thin batter. Pour 1/2 cup of the batter in center of the pan (tava) and quickly spread with outward spiral motion to form a thin pancake of about 8 inches in diameter. Place 2 tsps of potato masala in the center. Dot the edges with a tsp of ghee and cook until golden brown. Using a spatula carefully fold the dosa and remove from the pan. Repeat doing this with the remaining batter to make more dosas. Serve hot with chutneys or sambar