Preheat your oven to 350°F (175°C). Grease and flour a 9x5 inch loaf pan, or line it with parchment paper, leaving an overhang on the sides for easy removal.

In a large mixing bowl, whisk together the flour, granulated sugar, light brown sugar, baking soda, ground cinnamon, and salt. Set aside.

In a separate medium mixing bowl, mash the ripe bananas with a fork or potato masher until mostly smooth with a few small lumps remaining. Stir in the melted butter, egg, and vanilla extract until well combined.

Pour the wet banana mixture into the dry ingredients. Stir gently with a rubber spatula until just combined. Be careful not to overmix; a few lumps of flour are okay.

Gently fold in the diced rhubarb until evenly distributed throughout the batter.

Pour the batter into the prepared loaf pan and spread evenly.

Bake for 55-65 minutes, or until a wooden skewer or toothpick inserted into the center comes out clean. If the top starts to brown too quickly, you can tent it loosely with aluminum foil.

Once baked, remove the loaf pan from the oven and let it cool on a wire rack for 10-15 minutes before carefully removing the bread from the pan. Allow the bread to cool completely on the wire rack before slicing and serving. This helps prevent it from crumbling.