In a small bowl, combine first five ingredients. Place the lamb and beef in a large resealable plastic bag; add half of the marinade. Place vegetables in another large resealable plastic bag; add the remaining marinade. Seal bags and turn to coat; refrigerate 1 hour. , On eight metal or soaked wooden skewers, alternately thread lamb, beef, onions, red peppers and mushrooms. Grill, covered, over medium heat 5-6 minutes on each side or until meat reaches desired doneness and vegetables are tender. Serve with pita bread.