In large mixing bowl, blend ground beef and sausage; divide into eight patties.
On grill over medium heat, cook burgers on each side until no longer pink in center and internal temperature reaches 160° F, about 15 minutes.
Heat macaroni and cheese according to package instructions.
Place patties on buns and top each with 1/4 cup macaroni and cheese, 2 teaspoons barbecue sauce and fried onion straws.