Preheat oven to 350°F. Coat a 9x5-inch loaf pan (or muffin tin) with cooking spray.
In a large mixing bowl, stir together the mashed bananas, pumpkin, yogurt, melted butter, eggs, vanilla, and sugar, until well combined. (Or, put all the wet ingredients in the blender and blend until smooth. Use a spatula to help remove it in the next step.)
In another mixing bowl, whisk together the flours*, flaxseed, baking soda, salt, cinnamon, and nutmeg. Add this flour mixture to the wet mixture; stir just until blended. *Lightly spoon flour into dry measuring cups; level with a knife.
Pour batter into the loaf pan (or evenly into muffin cups). Use a scraper to make sure all the batter gets used.
For the bread, bake for about 45-55 minutes or until a wooden pick inserted in center comes out clean. It does brown quickly so don't let that trick you to taking it out too soon! Cover with foil if it starts to get too brown on top before the inside is done.
Remove from oven and let cool 10 minutes in the pan on a wire rack. Remove bread or muffins from pan and cool completely.