In a medium bowl, combine the flours and baking powder. Add the salted lukewarm water to the flour mixture, little by little, stirring, until the mixture has a dough-like consistency.
Divide the dough into tow portions. Flaatten one portion, sprinkle the white sesame seeds on top, an knead them in. Incorporate the black sesame seeds into the other half. Shape each piece of dough into a ball. Place the ballls in a plastic bag, and refrigerate them 1 hour.
Heat the oven to 400°F. On a lightly floured counter, roll each ball into a log about 2/3-inch in diameter. Cut each log into quarters, then each quarter into quarters. You want 32 disks of dough! Dredge the cut sides of the disks in flour to keep them from drying out.
Use a rolling pin to make each disk into a circle 2 1/2-inches in diameter. Arrange the thin disks on a baking sheet, and put the crackers into the hot over.
Japanese Rice CrackersBake the crackers until crisp and light golden, about 15 to 20 minutes, turning the crackers over once halfway through the baking. Watch them carefully! They are very thin and can burn very quickly; as you can see some of mine got a bit too brown.
Transfer the crackers to a rack to cool, then store them in a jar with a tight-fitting lid.
Serve the crackers with your favorite dip.