Form each lamb rack into half circle and place together to form a circle, bone-side to center. Tie to secure. Place on a rack in a roasting pan. Sprinkle lightly with salt and pepper. Roast in a 375ºF oven for 30 minutes.
In skillet, heat oil over medium-high heat. Sauté celery and onion 4 to 5 minutes, stirring occasionally. Stir in allspice and salt. Toss with cooked rice, cornbread crumbs, broth and cranberries. Spoon rice mixture loosely into center cavity of lamb crown. Roast 20 to 25 minutes more or until meat thermometer registers 145ºF for medium-rare, 160ºF for medium or 170ºF for well, and stuffing is heated through. Cover and let stand 10 minutes. Internal temperature will rise approximately 10 degrees. Garnish with oranges.
Cut between individual ribs to serve; serve with stuffing.