Prepare the Biga: In a large bowl, combine bread flour, warm water, and instant yeast. Use a wooden spoon to mix until you have a uniform mass, and no dry flour remains, about 1 minute. Cover tightly with plastic wrap and let it stand at room temperature overnight, allowing the flavors to develop.
Prepare the Soaker: In another bowl, mix the whole wheat flour, wheat germ, and whole milk. Stir with a wooden spoon until a shaggy mass forms, about 1 minute. Turn the dough onto a lightly floured work surface and knead until smooth, about 2 to 3 minutes. Return the soaker to its bowl, cover tightly with plastic wrap, and refrigerate overnight.
Prepare the Dough: Tear the refrigerated soaker into 1-inch pieces and place them in the bowl of a stand mixer fitted with the dough hook. Add in the biga, honey, salt, instant yeast, butter, and vegetable oil. Mix on low speed until a cohesive mass forms, about 2 minutes. Increase the speed to medium and knead until the dough is smooth and elastic, for about 8 to 10 minutes. It will be very soft and slack. Transfer it to a lightly greased bowl, cover tightly with plastic wrap, and allow it to rise at room temperature for about 45 minutes.
After the initial rise, gently press down on the dough's center to deflate it. Lightly flour your fingertips and begin folding the dough over itself, lifting and folding edges toward the center. Turn the bowl 90 degrees and repeat this process until you’ve completed 8 folds. Cover the bowl again and let it rise until it has doubled in volume, about 45 minutes.
Preheat your oven to 400 degrees F (200 degrees C). Adjust the oven racks to middle and lowest positions, placing a baking stone on the middle rack. Spray two 9×5-inch loaf pans with non-stick cooking spray. Divide the dough into two equal pieces, shaping each into an 8×17-inch rectangle. Roll each piece tightly into a cylinder, seam side down in the prepared loaf pans, pressing gently into the corners.
Cover the loaves loosely with plastic wrap and let them rise again at room temperature until almost doubled in size, 60 to 90 minutes. The dough should rise about 1 inch above the edge of the pan.
To create steam for a perfect crust, place an empty baking pan on the bottom rack of the oven. Boil 2 cups of water on the stove and set aside. Use a sharp knife to make a ¼-inch-deep slash lengthwise down the center of each loaf. Pour the boiling water into the empty baking pan before placing the loaves on the baking stone.
Reduce the oven temperature to 350 degrees F (177 degrees C). Bake until the crust is deep brown and the internal temperature reaches 200 degrees F (93 degrees C), about 40 to 50 minutes, rotating the loaves halfway through baking for even color.
Once baked, transfer the pans to a wire rack and cool for 5 minutes. Remove the loaves from the pans and place them back on the wire rack to cool to room temperature for about 2 hours. For storage, wrap the bread in plastic wrap for up to 3 days, or wrap in aluminum foil and freeze for up to 1 month.