In a small bowl, combine rosemary, garlic, lemon peel, and salt. Set aside.
Untie meat; trim fat from meat. Spread rosemary mixture over top of one of the loins. Place pancetta on top of the other loin. Reassemble meat, placing the pancetta-topped loin on top and positioning the rosemary mixture in the middle. Retie with 100-percent-cotton kitchen string. Wrap meat tightly in plastic wrap and chill for 8 to 24 hours.
Preheat oven to 325 degrees F. Place meat on a rack in a shallow roasting pan. Insert an ovenproof meat thermometer into center of meat. Roast for 2 to 2-1/2 hours or until meat thermometer registers 150 degrees F.
Cover meat with foil and let stand for 15 minutes. The temperature of the meat after standing should be 160 degrees F. Slice meat. Makes 12 to 15 servings.