Preheat oven to 375° Fahrenheit.
In a large saucepan, boil spaghetti until al dente. Remove from heat and drain.
While pasta is cooking, brown beef & sausage in a large skillet over medium heat. Add onion while browning meat. When brown, drain off any excess fat and return to skillet. Add tomato sauce, paste, Italian seasoning, garlic, salt & pepper. Mix well. Simmer for 5-8 minutes. Add cooked pasta. Mix well again and remove from heat.
While meat sauce is simmering, cut the Italian bread loaf in half using a bread knife. Carve out the middle of each half of the loaf. Remove any remaining bread to create a large hollow in each half. Brush insides of the hollows and sides of the halves with butter. Sprinkle with garlic powder. Toast in oven.
When bread is toasted, spoon the prepared spaghetti & meat sauce into the hollow of each half, filling the hollows completely. (You will have spaghetti & meat sauce left over). Top with cheeses- 1 of the divided halves of the cheeses for one loaf and one for the other.

Bake at 375° for 15-20 or until cheese is melted and golden. Remove from oven and gently cut with a bread knife to serve.

You will have spaghetti and meat sauce left over. You can freeze this and a have a small spaghetti meal later or make a 3rd garlic bread boat using a half of another Italian loaf.