Preheat oven to 350°F. Grease loaf pan with butter.
In bowl of stand mixer, combine 1 cup ripe mashed bananas and 1 1/2 cups canned pumpkin until well mixed. Add 2 large eggs, 1/4 cup lite olive oil, 1 teaspoon vanilla, 1/2 cup brown sugar, and 1/2 cup granulated sugar. Mix well on low speed. Scrape sides and mix again.
In a separate bowl, combine 2 1/2 cups all-purpose flour, 1 teaspoon baking powder, 1 teaspoon baking soda, 1/2 teaspoon salt, and 3 teaspoons 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 3 tablespoons all-purpose flour, 3 tablespoons brown sugar, 3 tablespoons cold unsalted butter, 1/2 teaspoon pumpkin pie spice, and 1/2 cup 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.