Using a knife, make an incision all the way down the back of each duck. Open up the ducks along the incision. Lay the breasts facing up and press to flatten.
Place the ducks in a large dish. Brush the entire surface of the ducks with two-thirds of the marinade. Cover and marinate for 8 to 12 hours in the refrigerator. Store the rest of the marinade in the refrigerator.
When ready to cook, preheat the oven to 180 °C (350 °F).
Place the ducks on a baking sheet lined with parchment paper, skin side up. Cover with a sheet of aluminum foil. Roast in the oven for 2 hours.
Remove the aluminum foil, and insert a cooking thermometer into the thigh, without touching the bone. Continue roasting for 30 to 40 minutes until the cooking thermometer reads 82 °C (180 °F).
Add the mayonnaise to the bowl containing the marinade and stir to combine. Serve with the ducks.