Remove the cavity fat from the ducks and set aside. Cut off and reserve the wing tips and second wing joints of both ducks. Leave the main wing bone attached. Quarter the gizzards and set aside. Reserve the livers.
To a saucepan add the duck fat and wing tips. Cut the second wing joints in half and add the pieces. Add livers, gizzards, and neck cut in thirds. Cook 20 minutes.
Add the celery, carrot and onion. Cook, stirring, about five minutes and drain well, discarding all fat. Return the solids to the saucepan and sprinkle with sugar. Cook, stirring frequently five to ten minutes (sugar supposed to be caramelized). Add the wine vinegar, cook briefly and add the chicken broth. Add salt and pepper to taste. Simmer one hour and 15 minutes.
Meanwhile, preheat oven to 450° F.
Sprinkle the ducks inside and out with salt and pepper. Truss the ducks with string. Place the ducks on a rack breast-side-up and bake 30 minutes. Carefully pour off the fat from the roasting pan.
Return the ducks to the oven and bake 15 minutes longer. Turn the ducks breast-side-down and bake 25 minutes longer. Pour off the fat from the pan.
Return the ducks to the oven breast-side up and bake 20minutes longer. Total baking time is one hour and 30 minutes.
Meanwhile, when the sauce has simmered the indicated one hour and 15 minutes, blend the cornstarch with water and stir it into the sauce. Strain the sauce through a sieve, preferably a chinois.
Return the sauce to a saucepan and stir in the Cassis. Swirl in the butter without boiling. Carve the duck. Serve with the sauce and wild rice.