Preheat: Preheat the oven to 375 degrees Fahrenheit. Butter an 8 or 9-inch loaf pan.
Make the batter: Mash the bananas in the bottom of a large bowl and measure them out. Whisk in the pumpkin puree, oil, brown sugar, eggs, and vanilla extract. In a separate bowl, whisk the baking soda, Pumpkin Pie Spice, kosher salt, all-purpose flour, and whole wheat flour. Then stir the dry ingredients into the batter with a spoon.
Bake: Pour the batter into the prepared loaf pan. Top with a sprinkle of turbinado sugar (or brown sugar) if desired. Bake 45 minutes, then add foil loosely on the top. Bake another 10 to 15 minutes until a toothpick inserted into the center of the loaf comes out clean (or the internal temperature is 200 degrees): the exact timing will depend on the size of the pan.
Cool: Cool in the pan for 15 minutes. Then run a knife around the edge and invert the loaf onto a cooling rack. Serve warm or at room temperature. Storage info: The bread lasts 4 to 5 days at room temperature wrapped in aluminum foil, refrigerated for 10 days, or frozen for 3 months (slice it into pieces and wrap it in plastic wrap then a freezer-safe bag or container).