Preheat your oven to 350°F (175°C). Grease a 9x5-inch loaf pan or line it with parchment paper for easy removal.
In a large bowl, whisk together the flour, baking soda, baking powder, cinnamon, nutmeg, and salt until well combined.
In another bowl, mix the granulated sugar, brown sugar, and vegetable oil until smooth. Add the pumpkin puree, grated zucchini, eggs, and vanilla extract, and stir until fully combined.
Gradually add the dry ingredients to the wet ingredients, mixing gently until just combined. Be careful not to overmix. If desired, fold in the chopped nuts and chocolate chips.
Pour the batter into the prepared loaf pan and smooth the top.
Bake for 60-70 minutes, or until a toothpick inserted into the center comes out clean.
Allow the bread to cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.