Combine pomegranate juice and sugar in a medium saucepan and bring to a simmer over medium heat.

Cook, stirring frequently, until glaze is thick and reduced to about 1 1/4 cups, 45 to 50 minutes, lowering heat as needed to prevent burning. Remove from heat.

Preheat the oven to 450°F.

Rub ducks inside and out with lemon juice and sprinkle inside and out with salt and pepper.

Place 1 onion and 2 rosemary sprigs in each duck cavity.

Tuck wings under backs of ducks and tie legs together with kitchen string.

Place on a rack in a large roasting pan. Roast until skin is browned, about 15 minutes.

Remove pan from oven, lower temperature to 375°F and prick duck skin all over with a fork.

Baste ducks with pomegranate glaze and continue roasting, basting with glaze about every 10 minutes. (If a lot of fat accumulates on the bottom of the roasting pan, spoon some of it out during cooking.)

Roast until an instant-read thermometer inserted into thickest part of thigh, without touching bone, reads 180°F and into breast reads 170°F, 1 hour to 1 hour and 15 minutes total.

Let ducks rest for 10 minutes to allow juices to settle and then baste again.

Carve ducks and transfer to a large serving platter. Garnish with pomegranate seeds, if using. Serve any leftover glaze on the side.