Beat 1/2 cup butter and shortening at medium speed with an electric mixer until fluffy; gradually add sugar, beating well until blended. Add egg yolks, 1 at a time, beating just until blended after each addition. Stir in vanilla.
Combine flour and baking soda; add to butter mixture alternately with buttermilk, beginning and ending with flour mixture. Beat at low speed until blended after each addition. Stir in pecans.
Beat egg whites at medium speed until stiff peaks form; fold one-third of egg whites into batter. Gently fold in remaining beaten egg whites just until blended. Use immediately.