1. Preheat oven to 325°. Beat butter and shortening at medium speed with a heavy-duty electric stand mixer until creamy. Gradually add granulated sugar, beating at medium speed until light and fluffy. Add eggs, 1 at a time, beating just until blended after each addition.
2. Stir together flour, baking powder, and salt. Add to butter mixture alternately with milk, beginning and ending with flour mixture. Beat at low speed just until blended after each addition. Stir in 2 tsp. orange zest, 1 tsp. lemon zest, and 1/4 cup fresh lemon juice. Pour batter into a greased and floured 10-inch (12-cup) Bundt pan.
3. Bake at 325° for 1 hour and 5 minutes to 1 hour and 15 minutes or until a long wooden pick inserted in center comes out clean. Cool in pan on a wire rack 10 minutes; remove from pan to wire rack, and cool completely (about 1 hour).
4. Whisk together powdered sugar, orange juice, and remaining 2 tsp. orange zest and 1 tsp. lemon zest until smooth. Spoon over cake.