Use the metal blade of your food processor to combine the flours, brown sugar, soda, salt and butter, about 10 pulses. Add the remaining ingredients, pulse 5 times, then process until the mixture forms a mass, about 20 seconds. Remove the dough, press it into a ball and divide it in half. Cover one half with plastic wrap. Preheat the oven to 325 degrees. Roll out the other half on a buttered rimless 14x17 inch baking sheet. Lightly flour the surface of the dough and roll until it is very thin and covers the baking sheet. Use the sharp side of a knife to cut the dough into 3 inch squares, then use the dull side of the knife to divide the squares in half. Prick evenly with a fork. Bake until evenly browned and firm to the touch, about 12-15 minutes. Repeat with the remaining dough on a second baking sheet. Follow the scored lines to cut the crackers apart while warm; they will crisp as they cool. Gently lift them from the baking sheet with a spatula. Separate the crackers and store in air- tight containers. Makes about 50 crackers.