Grease and flour four 9 inch cake pans. Preheat oven to 325 degrees F (165 degrees C). Sift together flour, cream of tartar, and baking powder four times. Set aside.
Beat 8 egg yolks with a rotary or electric beater until very thick, light, and creamy.
Add the 2 cups sugar, a few tablespoons at a time, and continue beating until mixture is smooth and pale yellow. Stir in 1 grated lemon rind and juice of 1 lemon. Quickly fold in the flour mixture until well incorporated.
In a large glass or metal mixing bowl, whip the 8 egg whites until peaks form. Fold 1/3 of the whites into the batter, then quickly fold in remaining whites until no streaks remain.
Spoon into cake pans and bake for 20 to 25 minutes or until cake begins to pull away from sides of pans. Loosen edges with a knife and turn out on cake racks to cool while you prepare the filling and frosting.
Mix 6 egg yolks with 2 cups sugar, grated rind of 2 lemons, and juice of 4 lemons. Cook over boiling water, stirring constantly, until sugar dissolves.
Add 1/2 cup butter and continue cooking, stirring constantly, for 20 minutes or until filling is smooth and very thick. Cool, then spread between layers of cooled cake.
To Make Lemon-Orange Frosting: Beat the 1/4 cup butter until it has the appearance of thick cream, stir in 6 cups confectioners sugar, a little at a time, and continue working until mixture is very smooth. Beat in 1 egg yolk and 2 tablespoons lemon juice. Stir in enough orange juice to make a spreadable frosting, then add grated rind of 1 lemon and 2 grated orange rinds.
