Marinade: About 8 hours or 1 day before serving, prick the skin of the duck all over with a fork. In a large pot over high heat, bring 8 quarts of water to a rapid boil and blanch the ducks for 5 minutes. Remove the ducks from the water and set aside. Combine all ingredients in a bowl. Brush on duck and let marinate at least 4 hours or overnight. Preheat oven to 400 degrees. Roast the ducks, breast side up, for 20 minutes, reduce the heat to 350 and continue cooking for another 1 hour. Remove from the oven and let rest at room temperature for about 20 minutes.
Pineapple-Red Onion Relish: Combine all in a medium bowl and season with salt and pepper to taste.
Assemble: Duck Flour tortillas, warmed Pineapple-Red Onion Relish Green onions
Carve the breast from each duck and slice very thin on the bias, allowing half a breast per person. Take a torilla, spread a little relish on it and place the sliced duck over that, top with a green onion, roll and eat.