Preheat oven to 375 degrees. Mix wheat flour, white flour, baking soda, and salt together in a medium bowl. Work butter into mixture with your fingers. Form a well in the middle of flour mixture and pour buttermilk into the well. Form your hand into a rigid claw and stir dough slowly but steadily in a spiral motion, starting in the middle and working outwards. Dough should be soft but not too wet or sticky. (Start with 1 3/4 cups buttermilk, gradually adding, if necessary, to achieve the right consistency.) Turn dough out onto a floured board. Flour your hands lightly, then shape dough into a flat rectangle about 2 inches high. Cut dough in half lengthwise with a wet knife, then gently push each half into a lightly greased glass or foil loaf pan. Bake for 50 to 60 minutes, or until the tops of the loaves are nicely browned. Makes 2 loaves.