Preheat oven to 450°. Pull out the loose fat inside the ducks. Dry them carefully and season both inside and out with salt and pepper. Fill them with the stuffing and sew or skewer the skin over the opening. Truss the legs and wings, place in a roasting pan, and with a sharp fork prick them all over to allow the fat to escape. Place the pan, uncovered, in the middle of the oven and cook for 30 minutes. Remove and discard all the fat. Lower the heat to 375°.
Meanwhile, heat the olive oil in a heavy-bottomed pan, add the onions and cook for 5 minutes. Add the tomatoes, ½ cup of the wine, the garlic cloves, thyme, bay leaves, salt, and pepper. Cook, covered, for 20 minutes over a low flame. Add the olives and set the sauce aside.
Again discard the fat in the pan and turn the ducks over on the other side. Pour the sauce over them and lower the heat to 350°. Cook for 50 minutes. (The entire cooking time is about 1¾ hours to 2 hours.)
Place the ducks on a warm platter. Add the remaining wine to the sauce in the pan and scrape up the bottom of the pan while cooking 2 minutes over high heat.
To serve, scoop out the stuffing and mound it in the center of a warm serving platter. Carve the ducks, arrange the pieces around the stuffing, and spoon the sauce over them. (There will be very little sauce, as the duck will have absorbed most of it while cooking.) Garnish with parsley.