Preheat oven to 325 degrees F (165 degrees C). Grease and flour two 9 inch pans. Sift together the flour, baking powder, and cream of tartar. Set aside.
In a medium bowl, beat together the 8 egg yolks and 2 cups sugar until thick and pale. Stir in the 2 teaspoons lemon zest and 2 tablespoons lemon juice. In a large glass or metal mixing bowl, beat egg whites and salt until soft peaks form. Fold whites into the egg yolk mixture alternately with the flour mixture.
Spread evenly into the prepared pans.
Bake for 25 to 30 minutes in the preheated oven, or until a toothpick inserted into the cake comes out clean.
Let layers cool in the pan for 15 minutes before inverting onto wire racks to cool completely. Using a long serrated knife, slice the layers in half horizontally.
To make the filling: In the top of a double boiler, combine the 1 1/3 cup sugar, 4 egg yolks, 2 1/2 teaspoon lemon zest and 1/3 cup lemon juice. Cook over high heat, stirring constantly, until the sugar is dissolved and mixture thickens.
Remove from heat, and stir in the butter. Cool to room temperature before filling cake.
To make the frosting: In a medium bowl, cream the 1/3 cup butter until light and fluffy. Gradually add the confectioners sugar and mix in the orange zest, orange juice, lemon zest and lemon juice. Finally, stir in coconut. Frost the outside of the filled cake.
