1. Preheat the oven to 400º.
2. Truss chickens: Tuck wings under the chicken, tie drumsticks together with kitchen string; tie another string around the back end of the chicken, securing the wings and thighs. Rub each chicken with 1 tablespoon olive oil, half the lemon zest, and half the thyme. Season each chicken with 1/2 teaspoon salt and 1/8 teaspoon pepper, and place them in a large roasting pan. Toss the shallots, potatoes, carrots, and turnips with the remaining 1 tablespoon olive oil and 1/4 teaspoon each salt and pepper in a large bowl; add to the roasting pan.
3. Roast for 30 minutes, brush both chickens with juices in pan, and stir vegetables. Continue roasting, stirring, and brushing every 15 minutes, for 1 hour longer, or until juices of thigh run clear when pierced with a paring knife, and vegetables are tender.
4. Remove the chickens from roasting pan, and let stand for 10 minutes before carving. Toss vegetables with pan juices. Drain excess juice.
5. Carve the chickens, arrange on a platter with vegetables, and serve hot.