To prepare sponge, lightly spoon 1 cup flour into a dry measuring cup; level with a knife. Combine 1 cup flour and next 4 ingredients (1 cup flour through 1 package of yeast) in a large bowl, stirring well with a whisk. Cover; chill 12 hours.
To prepare dough, let sponge stand at room temperature 30 minutes. Lightly spoon 3 1/2 cups bread flour and 1/2 cup semolina flour into dry measuring cups, and level with a knife. Add 3 cups bread flour, 1/2 cup semolina flour, 3/4 cup warm water, 1/2 cup warm milk, salt, and 1 package yeast to sponge, and stir well to form a soft dough. Turn dough out onto a floured surface. Knead until smooth and elastic (about 8 minutes), and add enough of remaining bread flour, 1 tablespoon at a time, to prevent dough from sticking to hands. Divide dough in half.
Working with 1 portion at a time (cover remaining dough to prevent drying), roll each into a 13 x 5-inch oval. Place, 3 inches apart, on a large baking sheet sprinkled with 2 tablespoons semolina flour. Taper ends of dough to form a "slipper." Sprinkle 1 tablespoon semolina flour over dough. Cover and let rise in a warm place (85°), free from drafts, 45 minutes or until doubled in size.
Preheat oven to 425º.
Uncover dough. Bake at 425º for 18 minutes or until the loaves are lightly browned and sound hollow when tapped. Remove from pan, and cool on a wire rack.