The night before baking the bread prepare this starter: In a medium sized non-metal mixing bowl dissolve 1/2 teaspoon active dry yeast in 1 cup warm water. Add 1 1/2 cup bread flour and mix well. Cover and let sit overnight at room temperature.
The next day: In a large mixing bowl, dissolve the 2 teaspoons yeast in the 2 cups warm water. Add the starter mixture, the whole wheat flour, 3 cups bread flour and the salt; stir until well combined. Add the remaining bread flour, 1/2 cup at a time, mixing well after each addition. When the dough has pulled together, turn it out onto a lightly floured surface and knead until smooth and elastic, about 8 minutes. Lightly oil a large bowl, place the dough in the bowl and turn to coat with oil. Cover with a damp cloth and let rise in a warm place until doubled in volume, about 1 hour.
Grease two 9x5 inch loaf pans. Deflate the dough and turn it out onto a lightly floured surface. Divide the dough into two equal pieces and form into loaves. Place the loaves into the prepared pans. Cover the loaves with a damp cloth and let rise until doubled in volume, about 45 minutes. Preheat the oven to 425 degrees F (220 degrees C).
Bake in the preheated oven for about 30 minutes or until the top is golden brown and the bottom of the loaf sounds hollow when tapped.