Preheat oven to 350 degrees F. Grease loaf pan with butter.
In bowl of stand mixer, combine mashed banana and pumpkin puree until well mixed. Add eggs, oil, vanilla, brown sugar, and white sugar. Mix well on low speed. Scrape sides and mix again.
In a separate bowl, combine flour, baking powder, baking soda, salt, and pumpkin pie spice. While mixer is running on low, spoon flour mixture in and continue to add until all ingredients have been combined. Do not over mix. Pour into prepared pan.
To make the streusel, combine flour, sugar, butter, pumpkin pie spice, and walnut pieces. Toss with fork to evenly distribute butter chunks. Sprinkle mixture over batter.
Bake in preheated oven for 55 minutes. Allow to cool in pan until cool enough to handle, then transfer to a cooling rack. Wrap tightly to store.