In a mixing bowl, dissolve yeast and 1 teaspoon sugar in 3/4 cup lukewarm water; let stand for 10 minutes. Heat 2 cups of water to boiling and pour over remaining sugar, salt, shortening and molasses in mixing bowl. Stir to dissolve shortening. Add remaining 2 cups of cold water and cool to lukewarm. When liquid is lukewarm add dissolved yeast and stir in mixed dark flours. Stir in enough all purpose flour to make a soft dough. Knead on floured surface until smooth and elastic, about 10 minutes. Place in greased bowl; turn once to grease top. Cover and let rise in warm place until double in size, about 1 hour.
Punch down dough. Divide into 4 parts. On lightly floured surface, roll or pat each half to a 14 x 7-inch rectangle. Starting with shorter side, roll up tightly, pressing dough into roll with each turn. Pinch edges and ends to seal. Place in greased 9 x 5 or 8 x 4-inch bread pans. Cover; let rise in warm place until double, about 30 to 45 minutes. Bake at 375 °F. for 15 to 20 minutes. Reduce heat to 325 °F for the remaining 25 minutes. Remove from pans; cool.