In a bowl, dissolve yeast and sugar in warm water.
Add 2 tablespoons oil, salt and enough flour to form a soft dough. Turn onto a floured surface; knead until smooth and elastic, about 6-8 minutes.
Place in a greased bowl, turning once to grease top. Cover and let rise in a warm place until doubled, about 1 hour.
Punch dough down. Turn onto a lightly floured surface; divide in half. Press dough onto the bottom and 1 in. up the sides of two greased 14-in. pizza pans.
Spread 3/4 cup pizza sauce over each.
Bake at 450° for 5 minutes.
Meanwhile, in a skillet, saute onions in remaining oil until tender.
Arrange tomato slices over pizzas.
Combine cheeses; sprinkle over tomatoes. Top with onions.
Bake 10-15 minutes longer or until cheese is melted. Warm remaining pizza sauce; serve with pizza.