Preheat oven to 325°F. Prepare pans with vegetable pan spray.
In large bowl, stir together flour, baking powder, baking soda and salt. Set aside. In large bowl, beat 3/4 cup (1 1/2 sticks) butter and sugar with electric mixer until light and fluffy. Add eggs one at a time, mixing well after each. Add vanilla and 1 teaspoon of each citrus zests; mix well. Add flour mixture alternately with orange juice, beating until just combined. Divide evenly into pans.
Bake 18-21 minutes, rotating pans half way between baking. Cakes are done when toothpick inserted in center comes out clean. Cool 5 minutes on cooling grid; remove from pans and cool completely.
For filling, beat shortening and 6 tablespoons butter in large bowl with electric mixer until light and fluffy. Gradually add sugar, one cup at a time, beating well on medium speed. Scrape sides and bottom of bowl often. When all sugar has been mixed in, icing will appear dry. Add 1/2 teaspoon of each citrus zests, juice and, if desired, liqueur; beat at medium speed until light and fluffy.
To assemble, cut the crowns off of each cake layer. Place first layer on cake board or serving plate. Spread a layer of icing to edges of cake. Top with second cake. Spread a thin layer of lemon curd to edges. Repeat, alternating layers of icing and lemon curd. On top cake, spread a layer of icing, followed by a layer of lemon curd.