Preheat oven to 350 degrees. Butter two 8 1/2-by-4 1/2-inch loaf pans. Line with parchment paper, allowing a 1-inch overhang on long sides. Butter parchment; set aside. Whisk together flours, baking powder, and salt in a medium bowl; set aside.
Put butter, granulated sugar, and vanilla into the bowl of an electric mixer fitted with the paddle attachment; mix on medium speed until pale and fluffy, 3 to 4 minutes. Mix in eggs, 1 at a time. Reduce speed to low. Add flour mixture in 2 batches, alternating with the milk.
Divide batter evenly between prepared pans. Bake until dark golden and a cake tester inserted into centers comes out clean, 55 to 60 minutes. Let cool on a wire rack 15 minutes. Lift cakes out; let cool completely on rack. Trim tops of cakes level. Cakes can be stored at room temperature, covered, up to 1 day.
Put cream and confectioners' sugar into the clean bowl of an electric mixer fitted with the whisk attachment; beat on medium-high speed until soft peaks form. Transfer whipped cream to a pastry bag fitted with a leaf tip (such as Ateco #113). Pipe a ruffled pattern on tops of cakes. Pile blueberries in center of 1 cake, and raspberries in center of the other.