Preheat the oven to 350 degrees F (175 degrees C). Grease three 9-inch round cake pans.
Make the cake: Combine buttermilk and baking soda together in a small bowl; set aside.
Beat butter, shortening, and sugar together in a large bowl with an electric mixer until light and fluffy. Mix in eggs, buttermilk mixture, vanilla, coconut, baking powder, and flour; stir until just combined. Pour batter into the prepared pans.
Bake in the preheated oven until a toothpick inserted into the center of the cake comes out clean, 30 to 35 minutes. Cool in the pans for 5 minutes; transfer to a wire rack and let cool completely, 20 to 25 minutes.
Make the frosting (while cakes are cooling): Beat cream cheese, butter, vanilla, and confectioners’ sugar in a medium bowl with an electric mixer until light and fluffy. Beat in cream, a little at a time, to attain the desired consistency. Stir in walnuts and coconut. Spread frosting between the cake layers and on the top and sides of cooled cake.