Preheat the oven to 350ºF. Coat a 9x5 loaf pan with cooking spray and set aside.
In a medium bowl, whisk together all-purpose flour, wheat flour, baking soda, salt, and cinnamon. Set aside. In the bowl of a stand mixer fitted with the paddle attachment, beat coconut oil and brown sugar on medium speed until well blended, about 1 minute. Add the eggs one at a time, beating well after each addition. Add the banana, yogurt, and vanilla. Pulse the mixer on low until just incorporated. Add the dry ingredients in three parts, pulsing after each addition until just incorporated. Do not overmix.
Pour half the batter into the prepared pan. Sprinkle evenly with brown sugar, cinnamon, and walnuts for the swirl. Carefully top with the remaining batter and smooth out the top. Bake 40-50 minutes or until a wooden toothpick inserted in the center comes out clean. Cool 15 minutes in the pan on a wire rack, then remove and cool completely on the wire rack.