In a large mixing bowl, combine flour, baking powder, baking soda, and salt.  Cut in margarine and shortening until it resembles coarse crumbs.  Make a well in the center and pour in the buttermilk substitute.  Stir until just combined.  Turn dough onto floured surface and fold dough over on itself about 6 times.  Press into a 1" thick round.  Cut out biscuits with a 2-inch cutter.  Bake at 450 for 15-17 minutes.  Makes about 10 biscuits.