Preheat the oven to 350°F, and line 2 (approximately 8½-inch) loaf pans with parchment paper. Set aside. In a large mixing bowl, combine both sugars with the oil and butter. Then mix in the mashed banana, sour cream, vanilla, cinnamon, allspice and salt. Use an electric mixer to blend until it's as smooth as possible. Add the eggs, and continue to mix until they're fully incorporated.
Now mix in 1½ cups of the gluten-free flour and baking soda, just until you no longer see dry spots.
In another bowl, gently toss the diced banana and chocolate chips with the remaining ¼ cup gluten-free flour. Then fold this into the batter. (This can help prevent them from falling to the bottom.)
Evenly divide the batter between the two parchment-lined pans, and place them both on a baking sheet.
Bake in the 350°F preheated oven until the cakes are golden brown along the edges, and are beginning to crack -- about 50 minutes.
Let the cakes cool for at least 1 hour in the pans before removing and slicing.