Dissolve baking soda into boiling water, set aside. Cream shortening and sugar, beat in eggs. Sift flour with salt, combine with creamed mixture. Blend in boiled water with baking soda, then buttermilk. Stir in both cereals, raisins and (optional) walnuts.
Refrigerate uncooked dough for up to 6 weeks.
Bake at 400 degrees for 20 minutes, or until knife inserted in center comes out clean.