In a large mixing bowl, with a handheld mixer or in a stand mixer, cream together the shortening and margarine until creamy and light.
Sift in the masa harina, baking powder, garlic powder and salt, then continue to beat for about 3 minutes, until a sandy looking mixture forms.
Pour the vegetable stock into the masa mixture and continue to beat until all the liquid is absorbed and a fluffy dough forms, about 5 minutes.
The tamale dough should have a moist -- almost like mashed potato consistency - and should be easily spread with a rubber spatula. If the mixture seems too wet, then sprinkle in another tablespoon or two of masa harina. If too dry, then drizzle a tablespoon of vegetable stock until the desired consistency is reached. Use this dough right away for making tamales.