Brown beef and turkey in a medium skillet; drain excess grease.
Add onion, chili powder, cumin, oregano and ¾ teaspoon garlic salt and cook for 10 minutes over medium heat, stirring occasionally. Stir in corn, tomatoes, chiles and olives. Cook over medium heat for 5 minutes, then turn into a lightly greased 11 x 7 inch pan.
Meanwhile, boil potatoes in lightly salted water for 10 minutes or until tender; mash and stir in remaining garlic salt and milk. Spread over meat mixture.
Bake at 400°F for 15 minutes or until the potatoes are lightly browned. Top with cheese and bake for 5 minutes more.