Preheat the oven to 350°F. In a large mixing bowl, mash the ripe plantains until mostly smooth. Add the sweet potato puree and mix until well combined.
Add the sugar, melted butter, beaten eggs, sour cream, and vanilla extract one at a time, using a hand mixer or whisk to incorporate each ingredient into the plantain and sweet potato mixture before adding the next.
In a separate bowl, sift together the all-purpose flour, baking soda, baking powder, salt, and mixed spice.
Gradually incorporate the dry ingredients into the wet ingredients. Add half of the flour mixture and gently combine using a rubber spatula or a hand mixer on low speed. Add the remaining flour mixture and mix just until combined. Be careful not to over mix.
Fold in the chopped walnuts, reserving 2 tablespoons for topping.
Prepare two 8 x 3½-inch loaf pans or one 9 x 5-inch loaf pan with baking spray. Divide or pour the batter into the prepared pan(s), then sprinkle the reserved walnuts over the top.
Bake for 50 to 60 minutes, or until a toothpick or thin knife inserted into the center comes out clean or with only a few moist crumbs. Baking time may vary depending on the size of the loaf pan.
Allow the sweet potato plantain bread to cool in the pan for 15 minutes. Remove it from the pan and transfer it to a wire rack.Cool for approximately 45 additional minutes before slicing.