Whisk whole-wheat flour, all-purpose flour, baking soda and salt in a large bowl. Make a well in the center and add 1 1/2 cups buttermilk. Stir with a wooden spoon until all the flour is incorporated. The dough should be soft but not too wet. If necessary, add additional buttermilk, 1 tablespoon at a time, until the dough absorbs all of the flour. Use your hands to help the dough come together, if necessary. Using an ice cream scoop or a 1/4-cup measure, divide the dough among the prepared muffin cups.