Pre-heat oven to 350 degrees Fahrenheit. Cut the baguettes in half, and then cut lengthwise. Remove some of the bread from the inside to make a well for the meatballs. Set halves of bread, soft side facing up, on a baking sheet.
Spread butter on the bread, and bake in the oven for 15-20 minutes.
Meanwhile in a medium saucepan, heat pasta sauce and meatballs on medium heat. Cover and simmer for approximately 10 minutes, or until heated. Add salt and pepper to taste.
Remove bread from the oven and let cool. Spoon warm meatballs and sauce into each sandwich, and top with mozzarella cheese.