Preheat oven to 350°. Grease and lightly flour a 15" X 10" X 1" jelly roll pan.
Combine the flour, wheat flour, brown sugar, baking powder, baking soda, cinnamon, nutmeg, salt, oil, water, and eggs in a large bowl and mix well. Spread into pan.
Bake for 22-28 minutes or until toothpick inserted in center comes out clean. Cool completely.
Mix the powdered sugar, margarine, vanilla, and enough yogurt so that the frosting is a spreadable consistency. Beat with an electric mixer until smooth. Frost cooled bars, garnish with nutmeg, if desired. Refrigerate to set frosting.