Preheat oven to 400 degrees. Cut the squash in half. Scoop out and discard the seeds. Place, cut side down, in a baking pan. Add 1/2" of hot water. Bake, covered, until the squash is fork-tender (30-40 minutes). Scoop out the cooked squash and puree in a blender until smooth (you should wind up with 1 1/2 cups of squash puree for 2 loaves). Stir the yeast and sugar into the warm (110-115 degrees) water. Let stand for 5 minutes. Stir in the buttermilk and butter. Stir in the squash puree, carrots, orange rind, all-purpose flour, 2/3 of the whole-wheat flour, the spices and salt. Mix in enough of the remaining whole-wheat flour to form a soft dough. Knead on a floured surface until smooth and elastic (about 10 minutes). Place in a greased bowl and let rise, loosely covered, in a warm place until doubled in size (40-60 minutes). Punch down. Knead on a floured surface for about 5 minutes. Shape into loaves and place in greased (8 1/2"x4 1/2") loaf pans. Let rise, loosely covered, until doubled in size. Bake until loaves are golden and sound hollow when tapped (about 40 minutes). Cool on wire racks.