For the biga, stir the water, bread flour, whole wheat flour and yeast together in a bowl with a wooden spoon until evenly blended. Cover and set this starter aside on the counter for 12 to 18 hours.

For the dough, stir the biga and water in a large bowl so that the biga breaks down a little and becomes softer. Add the flour and yeast and mix by hand with a wooden spoon. Let the dough sit, uncovered, for 15 minutes, then add the salt and stir in vigorously until the dough feels stretchy.

Cover the bowl with plastic wrap and let sit on the counter for 2 hours, until doubled in size. After the first hour, knock the dough down by patting it with a wooden spoon 4 to 5 times and then re-cover it.

Line two baking trays with parchment paper and dust them with flour. On a well-floured work surface, turn the dough out. Using your hands and/or a bench scraper, and lots of flour, divide the dough in half and shape each of the pieces into a rough 8 × 12-inch rectangle about 1½ inches tall. Transfer each piece to a baking tray and let the dough rest for 30 minutes, uncovered.

Preheat the oven to 450°F (230°C). Dust the tops of the loaves with flour and bake the bread for about 30 minutes, until an even golden brown. Turn off the oven and leave the bread in for 10 minutes. Transfer the loaves from the baking trays to a cooling rack for at least 15 minutes before slicing.