To make the five-spice rub, put the fennel, cinnamon, star anise, Szechwan peppercorns (or cardamom), cloves, and chipotle pepper in a small pan over medium heat and toast for a few minutes until aromatic, shaking the pan. Let cool, then grind the toasted spices in a spice mill or clean coffee grinder. Transfer the ground spices to a bowl. Add the ginger, garlic, brown sugar, salt, and pepper. Stir to combine well.
Pat spice rub generously all over the skin of the ducks, place them on a rack over a roasting pan, and refrigerate them, uncovered, for at least 2 hours, or, preferably overnight.
Preheat a grill.
When you are ready to cook the ducks, place them on a rotisserie over a grill with a drip pan set directly under the ducks. Keep the fire fairly low to avoid flare ups. Cook the ducks until the juices run clear and an instant read thermometer inserted into the thigh reads 175 degrees F, about 2 to 2 1/2 hours. Remove the ducks from the rotisserie and carve them into breast and leg-thigh portions.
Note: If you don't have a rotisserie, you can roast the ducks in the oven on a rack in a roasting pan. Roast the ducks at 500 degrees F for 30 minutes, then turn the heat down to 350 degrees F and continue roasting for another 50 minutes or more until the juices run clear and an instant read thermometer inserted in the thigh reads 175 degrees F. (It takes longer to cook the ducks on a rotisserie over a grill because you have to keep the fire low to avoid flare-ups from the dripping duck fat.)