With the rack in the middle position, preheat the oven to 375°F (190°C).
In the bottom of a large roasting pan, mix the carrots, onions and garlic with the oil. Season with pepper. Place a roasting rack on top of the vegetables. Place the chickens, breast side up, on the rack. Spread half of the herbes salées in the cavities of the chickens and rub the other half over the breasts and thighs. Let marinate for 15 minutes. Transfer to the oven and roast for 1 hour.
Remove from the oven. Place the rack with the chickens on a baking sheet. Add the chickpeas, zucchini, tomatoes, tomato paste, harissa and 1 1/2 cups (375 ml) of the chicken broth to the roasting pan. Stir well. Place the rack with the chickens back in the roasting pan and return to the oven. Roast for 45 minutes or until a thermometer inserted into the thigh, without touching the bone, reads 180°F (82°C). Let rest for 10 minutes.
In a pot, bring the remaining broth to a boil. Remove from the heat and add the couscous. Cover and let sit for 5 minutes. Fluff the couscous with a fork.
Carve the chicken by removing the legs and breasts. Place the meat on a serving platter and garnish with the vegetables. Serve with the couscous.