In an extra-large bowl stir together the warm water, yeast, and sugar; let stand about 10 minutes or until mixture is foamy. Stir in yogurt, warm milk, and salt. In a medium bowl stir together all-purpose flour and whole wheat flour. Add 2 1/2 cups of the flour mixture to yeast mixture; stir just until combined. Reserve the remaining 1 1/2 cups flour mixture. Cover bowl with plastic wrap; let stand at room temperature for 2 hours.
Stir 1 1/4 cups of the remaining flour mixture into dough. Sprinkle a work surface with the remaining 1/4 cup flour mixture; turn dough out onto the floured surface. Gently knead for 2 1/2 to 3 minutes or until flour is incorporated (dough will be sticky). Cover dough on work surface and let stand at room temperature for 30 minutes more.
Lightly coat a 6-quart slow cooker with cooking spray. Line the bottom of the cooker with parchment paper; coat paper with cooking spray and sprinkle with cornmeal. Dust top of dough lightly with additional flour. Use a pastry scraper or wide spatula to loosen dough from surface. Place dough in the center of the prepared cooker, keeping dough in a round shape and gently handling dough so you don't disturb the air bubbles that have formed.
Cover and cook on high-heat setting for 2 1/4 to 2 1/2 hours or until an instant-read thermometer inserted near the center registers 200 degrees F, giving the crockery liner a half-turn after 1 1/2 hours of cooking, if possible. (Do not lift the lid.)
Preheat broiler. Remove bread from cooker; peel off parchment paper. Place bread on an ungreased baking sheet. Broil 4 to 6 inches from the heat for 2 to 3 minutes or until bread is golden and surface is no longer moist. Transfer to a wire rack; cool completely before slicing.