Preheat oven to 325 degrees. In the bowl of a stand up mixer, combine spice cake mix, pumpkin pie mix, eggs, milk, and baking soda. Beat with the flat mixer attachment until well blended. Or use an electric mixer until well blended. Spread batter in a 13x9 baking pan. Bake for 60 minutes or until a toothpick inserted near the center comes out clean.