Put a rack in the middle of the oven and preheat oven to 350 degrees. Generously butter two 8- x 4-inch loaf pans and dust with flour, knocking out excess.

Sift together flour, baking powder, baking soda, and salt into a medium bowl. Beat together butter and sugars in a large bowl with an electric mixer at high speed until pale and fluffy, about 2 minutes. Reduce speed to medium and add eggs one at a time, beating well after each addition. Add vanilla, zest, bananas, and sour cream, beating until just combined. Reduce speed to low, add flour mixture, and mix until just combined. Stir in nuts and coconut.

Divide batter between buttered loaf pans and smooth tops. Bake until a wooden pick or skewer inserted in center of bread comes out clean, 45 to 50 minutes. Remove bread from pans and cool completely, right side up, on rack.

Makes 2 loaves.