Prepare the Doughs: In separate bowls, mix the dry ingredients for both the light and dark rye doughs. Add wet ingredients and knead until doughs are smooth and elastic.
First Rise: Let both doughs rise in oiled bowls for about 1 hour, until doubled in size.
Shape the Doughs: Punch down the doughs and roll them out into 8x12-inch rectangles.
Create the Marble Effect: Cut each dough into 6 strips and alternate them to form a striped rectangle. Stack and roll the dough into a log, shaping it into a loaf.
Second Rise: Let the loaf rise for 45 minutes.
Preheat and Bake: Preheat the oven to 375°F, brush the loaf with egg white, and bake for 30-35 minutes.
Cool: Let the loaf cool on a wire rack for at least 1 hour before slicing.