Combine milk, sugar, shortening and salt. Heat until warm (100° to 110°F); pour into large mixer bowl. Add yeast and stir. Let stand for 5 minutes. Add 3-1/4 cups bread flour; beat with electric mixer on low for 30 seconds. Beat on medium-high for 3 minutes. Remove 2-1/2 cups of the batter and place in large mixing bowl.
To the 2-1/2 cups of batter, stir in as much remaining bread flour as possible to make a soft dough. Knead on lightly floured surface 8 to 10 minutes until smooth and elastic, adding additional flour as needed. Shape into a ball. Place in a greased bowl, turning once to coat. Cover; set aside.
To remaining batter, stir in molasses, whole wheat flour and flax seed. Knead on lightly floured surface 8 to 10 minutes, adding additional bread flour as needed, until smooth and elastic. Shape into a ball. Place in a greased bowl, turning once to coat. Cover; set aside.
Let both portions of dough rise in a warm, draft free place until double, about 60 to 75 minutes. Punch each down and divide each portion in half. Cover. Let rest 10 minutes. Grease two 9 x 5-inch loaf pans.
Roll out half the light dough and half the dark dough, each into a 12 x 8-inch rectangle. Place dark dough on top of light dough. Beginning with the short side, roll up tightly and shape into a loaf. Seal seams with fingertips. Repeat with remaining dough sections to make 2nd loaf.
Place seam side down, in prepared pans. Cover and let rise until double, about 45 to 60 minutes. Bake in a preheated 375°F oven for 30 to 35 minutes, or until internal temperature reaches 190°F. Remove from pans immediately. Cool on wire rack.