To make the cake, sift together flour and 1/2 cup sugar. Beat egg whites in a large bowl until foamy; add salt and cream of tartar and vanilla. Beat until soft peaks form. Sprinkle in 1 cup sugar, 1/4 cup at a time, beating until just blended. Fold flour mixture into egg whites, 1/4 cup at a time, using a rubber spatula.
Beat egg yolks with orange rind, orange juice, and remaining 2 tablespoons sugar until thick and light. Stir 1/3 of the egg white mixture into the egg yolk mixture.
Scrape yellow and white batters into an ungreased 10-inch tube pan to give a marbled effect with white layer on top. Bake 30 to 35 minutes, (check about 20 min into the cooking) or until done. Invert pan; let cool. Remove cake from pan.
To prepare frosting, combine butter, salt, orange rind and 1 cup powdered sugar in a mixing bowl. Beat with a mixer at medium-high speed until fluffy. Add remaining sugar and orange juice alternately, beating until smooth and spreadable. Beat in lemon juice.