Preheat oven to 325°F. Line two 9x5-inch loaf pans with a strip of parchment paper and grease with nonstick baking spray.
In a large bowl, combine flour, brown sugar, baking powder, baking soda, cinnamon and salt.
In another bowl, whisk together the zucchini, banana, oil, eggs, and vanilla extract. Add to the dry ingredients and mix just until no streaks of flour remain. Stir in walnuts. Do not overmix.
Divide batter between the two prepared loaf pans. Bake for 40-50 minutes, or until bread is golden brown and a toothpick inserted into the center comes out clean with a few crumbs.
Let loaves cool in the pan for 5-10 minutes, then transfer to a cooling rack to cool completely.
Store loaves at room temperature for several days. Or double wrap in plastic wrap and freeze for up to 2 months or more.
Calories: 2360kcal, Carbohydrates: 312g, Protein: 36g, Fat: 111g, Saturated Fat: 10g, Cholesterol: 246mg, Sodium: 1272mg, Potassium: 1623mg, Fiber: 13g, Sugar: 126g, Vitamin A: 680IU, Vitamin C: 32mg, Calcium: 344mg, Iron: 14mg