Beat butter and shortening at medium speed with an electric mixer until fluffy; gradually add sugar, beating well. Add egg yolks, 1 at a time, beating until blended after each addition. Add vanilla; beat until blended.
Combine flour and soda; add to sugar mixture alternately with buttermilk, beginning and ending with flour mixture. Beat at low speed until blended after each addition. Stir in 1 cup flaked coconut.
Beat egg whites until stiff peaks form; fold into batter. Pour batter into 3 greased and floured 9-inch round cakepans.
Bake at 350° for 25 minutes or until a wooden pick inserted in center comes out clean. Cool in pans on wire racks for 10 minutes; remove from pans, and cool completely on wire racks.
Spread 3/4 cup chilled Fresh Orange Curd between layers; spread remaining Fresh Orange Curd on top of cake. (The Fresh Orange Curd layer on top of cake will be very thick.) If desired, loosely cover cake, and chill 8 hours. (Chilling the cake with the curd between the layers helps keep the layers in place and makes it much easier to spread the frosting.) Spread 3 cups Pecan-Cream Cheese Frosting on sides of cake, reserving remaining frosting for another use. Sprinkle 1/2 cup toasted coconut over top of cake, if desired. Arrange Glazed Pecan Halves around top edge of cake, if desired. Store in refrigerator until ready to serve. Arrange Boxwood Garland around bottom edge of cake before serving, if desired.
Note: Cake may be frosted with Pecan-Cream Cheese Frosting immediately after adding the Fresh Orange Curd, but the cake layers will not be as steady.
You'll need to start this cake a day ahead because the Fresh Orange Curd must chill for 8 hours.