Place cake flour, all-purpose flour and salt in food processor bowl with metal blade. Cover and process 2 seconds to aerate. Divide shortening into 10 pieces and distribute evenly over dry ingredients. Process 15 to 20 seconds, or until fine crumbs form. In small bowl, sprinkle yeast and sugar over warm water; stir until dissolved. While processor is running, pour yeast mixture through feed tube in a steady stream. Process until mixture forms a ball; continue processing for 2 minutes. Carefully turn out dough onto lightly floured surface. With floured, rolling pin, roll out dough as thin as possible into a rectangle. Fold each long side to center; then fold each end to center to form a smaller rectangle. Repeat rolling and folding twice more, adding flour to surface as necessary to keep dough from sticking. Roll out dough into an 8 x 6' rectangle. Cut rectangle into 48 (1") squares. Place squares 1" apart, on greased large baking sheet. With fork, prick each square once. Bake in 375 oven 20 to 25 minutes or until lightly browned. Immediately remove from baking sheet. Cool on racks. Store in airtight container up to 1 week. Makes 4 dozen crackers.