Preheat oven to 350 degrees. Line 24 muffin-tin cups with paper liners. Beat bananas with a mixer on medium speed until mashed. Beat in sugar, oil, and egg until smooth. Reduce speed to low. Add all-purpose and whole-wheat flours, baking powder and baking soda, and salt; beat until smooth. Beat in yogurt and vanilla until combined. Fold in walnuts. Fill muffin liners 3/4 full; bake until a tester comes out clean, 20 minutes.