Preheat oven to 375 degrees F.

Rub the ducks lightly with olive oil and liberally sprinkle with salt and pepper. Stuff each bird with equal amounts of the onions and garlic. Arrange the ducks, not touching, on a rack in a roasting pan, breast side up. Bake until the skin is golden,  the juices run clear, and the internal temperature reaches 165 degrees F, about  2 1/2 hours.