Dissolve yeast and sugar in warm water in a large bowl; let stand 5 minutes. Stir in yogurt, honey, and butter. Add whole-wheat flour, walnuts, 1/2 cup pumpkin seeds, 1/2 cup sesame seeds, and salt to yogurt mixture; stir well. Add 1 1/2 cups all-purpose flour, stirring to form a soft dough. Turn dough out onto a floured surface; knead until smooth and elastic. Add enough flour, 1 tablespoon at a time, to prevent dough from sticking to hands.
Place dough in a large bowl coated with cooking spray; turn to coat top. Cover with plastic wrap; let rise in a warm place (85°), free from drafts, 1 1/2 hours, or until doubled in size. (Press fingers into dough. If indentation stays, dough has risen enough.) Punch dough down; divide in half. Shape each half into a 7-inch round loaf. Place loaves diagonally 3 inches apart on a baking sheet coated with cooking spray. Cover; let rise 40 minutes, or until doubled in size.
Preheat oven to 350°. Brush egg white over loaves, and sprinkle with 2 teaspoons each pumpkin and sesame seeds. Bake at 350° for 25 minutes, or until the loaves are browned on bottom. Remove from pan, and cool on wire racks.