Mix both dals and wash several times until the water appears clear. Soak dal in 4 cups of water for at least six hours. Drain the water. In a blender, blend the dal to very creamy texture. Use only as little water as needed to blend. Using too much water for blending will not give the desired creamy texture.
Whip the dal batter, stirring with a spoon in a circular motion two to three minutes, until batter is light and fluffy. Add a few spoonfuls of water as needed.
Cover the bottom of a flat frying pan with ½-inch of oil and heat over medium high heat. Test the oil by dropping a little batter into it. When the oil sizzles, it is ready.
Place one tablespoon of batter into the oil. The oil should not be deep enough to cover the batter; the vadas will be softer if they are shallow-fried. The vadas will begin to expand slowly, without changing color.
Fry the vadas in small batches, until golden all around.
After you have made all of the vadas, place them in lukewarm water and soak for about 20 minutes. Squeeze the vadas gently to remove the water, being careful not to break the vadas. Set aside.
Whip the yogurt until smooth. Add a little milk as needed to make the consistency similar to pancake batter or lassi. Add about one teaspoon salt, to taste.
Soak vadas in the yogurt batter for at least half an hour, giving the vadas time to absorb the yogurt.
Slowly pick up each vada and transfer to a serving dish. Pour a few more spoons yogurt over the Vadas.
Just before serving, sprinkle some salt as needed, ground roasted cumin seeds, red chile powder, and finish off with a drizzle of tamarind chutney.