Heat oven to 325 degrees. Sift together flour, baking powder, and salt twice. Set aside.
Place egg yolks in the large bowl of an electric mixer. Beat on medium high until pale and foamy, 3 to 5 minutes. Gradually add 1/2 cup of the sugar, beating until very pale and puffy, 5 to 7 minutes.
Add oil in a steady stream. Add zests; beat for 1 minute. Reduce speed to medium low, and add flour mixture alternately with juices, flour in 4 parts and liquid in 3 parts, beginning and ending with the flour. Set aside.
In a clean bowl, whip egg whites on low speed until foamy. Add cream of tartar, and increase speed to medium high. Beat until soft peaks form. Gradually add remaining sugar, and beat for 1 1/2 minutes, or until whites are glossy and stiff peaks form.
Stir a quarter of the whites into the flour-yolk mixture. Fold in remaining whites carefully, trying not to deflate the batter. Gently pour into an ungreased 7-inch tube pan. Bake until cake is golden brown and a cake tester comes out clean, about 40 minutes.
Remove cake from oven, and invert immediately. Cool completely in pan. Run a knife around sides to loosen, and remove outer part of pan. Run a knife along bottom and around tube to loosen and remove tube. Turn upside down onto a cake round slightly smaller than the cake, and slice horizontally into 3 layers using a serrated knife.
Spread half the filling on the bottom layer, and cover with the middle layer. Spread remaining filling on middle layer, and cover with top layer. Refrigerate overnight, or until set. Brush off excess crumbs with a dry pastry brush.
Heat oven to 400 degrees. Prepare meringue, and spread it all over cake, swirling with an icing spatula as desired. Set cake, still on cake round, on an oiled baking sheet, and place in oven. Bake, watching carefully, until meringue is brown around edges and beginning to brown elsewhere. Transfer to a serving plate and serve as soon as possible. Slice using a knife dipped in hot water and wiped dry.