To make the light dough: In the bowl of a stand mixer with the paddle attachment, mix the bread and rye flours, salt, yeast, caraway seeds (if using), water, oil and molasses on low until combined. Increase the speed to medium-low and mix until well combined and the sides of the bowl are clean. Switch to the dough hook and knead on medium-low speed until the dough is smooth and tacky. (If mixing by hand, use a wooden spoon to mix and knead with your hands on surface lightly coated with vegetable oil.) If the dough is too sticky, knead in more bread flour 1 tablespoon at a time. If it's very stiff, knead in water 1 tablespoon at a time.
Spray a medium bowl with baking oil. Form dough into a ball. Place in bowl, then turn over so that all surfaces are coated. Cover tightly with plastic wrap. Let sit at room temperature until doubled in size, about 1 1/2 hours.
To make the dark dough: Immediately after making the light dough, repeat the process to make the dark dough, adding more water if needed to make a dough with the same feel as the first dough. Stretch and fold the dough into a ball and let it rise in another lightly oiled bowl as described above.
To shape the dough: When both doughs have doubled in size, divide each into 4 equal pieces. Generously coat the inside of 2 (8 1/2- by 4 1/2-inch) loaf pans with cooking spray.
Rub a little vegetable oil on a work surface. Using your hands or a rolling pin, gently shape each piece into a 6-inch square that's about 1/4-inch thick.
Make a stack of 4 dough squares, alternating light and dark doughs. Firmly pat down the stack with the palm of your hand so that all 4 pieces adhere to one another.