Preheat oven to 350 degrees F. Lightly oil bottom and 1/2-inch up the sides of an 8x4x2-inch loaf pan with olive oil. In a large bowl combine flours, sugar, flaxseed, baking powder, baking soda, and salt. Make a well in the center of the flour mixture; set aside.
In a medium bowl whisk together bananas, peanut butter, yogurt, eggs, and olive oil. Add banana mixture all at once to flour mixture. Stir just until moistened. Fold in chocolate pieces. Spoon batter into prepared pan.
Bake 50 to 60 minutes or until a wooden toothpick inserted near center comes out clean. Cool in pan on a wire rack for 10 minutes. Remove from pan. Cool completely on rack. Wrap; store overnight before slicing.