In a small bowl, combine first 5 ingredients, and set aside for topping. In a medium mixing bowl, stir together the flours, baking powder, cinnamon, baking soda and salt. Set aside. Combine yogurt and milk In a large mixing bowl, beat together brown sugar and butter. Add eggs and vanilla. Beat until fluffy. To this >> mixture, alternately add the 2-flour mixture and the yogurt mixture, beating until well blended. Fold in the rhubarb. Pour batter into a greased and floured 9x13-inch baking dish or pan. Sprinkle topping over batter. Bake at 350 degrees for 45 to 50 minutes, or until done. Serve warm or cool.