In a large bowl, combine the beef, sausage and spaghetti. Add the cheese, marinara sauce, 2 teaspoons garlic and the Cajun seasoning. Mix well and form into 6 oval-shaped patties.
Preheat a stovetop grill over medium heat. In a small bowl, mix the butter and remaining 2 teaspoons garlic. Spread the garlic butter on both sides of each bread slice and grill until toasted, 1 to 2 minutes per side. Set aside. Grill the patties for 6 to 7 minutes per side. Top 6 bread slices with a patty and set the remaining bread slices into place.