Preheat to 350°F (175°C). Grease or line a loaf pan.
Peel, cube, and boil until tender (10–12 min). Drain, mash, and let cool.
Mash bananas in a bowl, then mix in cooled sweet potato.
Whisk in sugars, oil, eggs, and vanilla until smooth.
In another bowl, whisk flour, baking soda, baking powder, salt, cinnamon, and nutmeg.
Fold dry ingredients into the wet until just combined.
Stir in nuts or chocolate chips if desired. (Optional)
Pour into loaf pan. Bake 50–60 minutes, checking with a toothpick at 50.
Cool in pan for 10 minutes, then on a rack. Slice when fully cooled.