Place the first 10 ingredients in a large roasting pan; cover and bake at 325° for 3-4 hours or until pork is tender. Remove pork roast; set aside.
Bake chuck roast, covered, 40 minutes longer or until beef is tender. Cut pork and beef into bite-size pieces. Strain pan drippings and set aside 1/2 cup sauce.
In a large Dutch oven, saute onion in butter until tender. Stir in the ketchup, vinegar, sugar, Worcestershire sauce, brown sugar, seasonings and reserved pan drippings. Add meat; bring to a boil. Reduce heat; cover and simmer for 15 minutes, stirring occasionally. Serve on buns. Yield: 16-20 servings.