Step 1

Prepare the Cake: Preheat oven to 350°F. Grease and flour a 12-cup Bundt pan.

Step 2

Beat sugar, butter, and shortening in a large bowl with an electric mixer on medium speed until light and fluffy, 5 to 6 minutes. Add eggs, 1 at a time, beating well after each addition.

Step 3

Whisk together flour, baking powder, salt, and baking soda in a bowl. Alternately add flour mixture  and buttermilk to butter mixture in 5 additions, beginning and ending with flour, beating until blended after each addition. Beat in vanilla and lemon extracts. Transfer batter to prepared pan; smooth top. Bake in preheated oven until a wooden pick inserted in the center comes out clean, 45 to 50 minutes. Cool in pan on a wire rack 10 minutes; remove from pan, and cool completely on rack, 30 to 40 minutes.

Step 4

Prepare the Vanilla Icing: Whisk together powdered sugar, milk, and vanilla extract in a bowl until smooth. Drizzle icing over Cake, letting it drip down sides. Refrigerate 10 minutes to allow icing to set.

Step 5

Prepare the Lemon-Orange Icing: Whisk together powdered sugar, fresh lemon and orange juices, and yellow food coloring gel in a bowl until smooth. Drizzle Lemon-Orange Icing over Vanilla Icing, letting it drip down sides. Let stand until set, about 20 minutes. Garnish with thin lemon or orange  slices, if desired.