In small bowl, combine soy milk with lemon juice. Set aside. (Mixture will thicken and resemble buttermilk.)
In medium bowl, combine flours, sugar, baking powder, baking soda and salt. Set aside.
In large bowl, combine oil and mashed sweet potatoes. Alternately add dry ingredients and soy milk mixture to sweet potatoes to form a soft dough.
Turn out onto a lightly floured work surface and knead briefly. (Don’t overwork or biscuit will be tough.) Pat dough into a 12 x 9-inch rectangle about 1/2 inch thick. Cut dough with a floured round biscuit cutter. Place biscuits on a baking sheet, spacing about 1/2 inch apart. Bake until golden, about 8 to 10 minutes. Serve warm.