Remove leg/thigh sections from ducks and place in a large non-reactive (glass, plastic, ceramic) container. Remove breast halves and place in container. In another bowl, whisk together half of the oil, orange juice concentrate, brandy, broth, peppercorns and salt. Pour half of the mixture over duck pieces and toss to coat. Reserve other half. Cover and refrigerate duck for 2- 6 hours. Remove breasts from marinade and set aside. Transfer duck legs with marinade to a greased baking dish, cover and place in a preheated 375 degree oven for 2 hours or until legs are tender. Heat remaining oil in a large skillet over medium-high heat. Brown breasts evenly, but not past rare. Remove breasts from pan and add reserved marinade, garlic and onion. Simmer for 10 minutes. Return breasts to pan, add cooked legs and warm to serving temperature. Remove duck and arrange on platter, Whisk butter into sauce in pan until melted and spoon sauce over duck.