Spread 1 tablespoon of the oil in the bottom of a roasting pan. Spread the fennel in a single layer and drizzle with 1 tablespoon of oil.
Salt the body cavities of the chickens and stuff 6 lemon quarters into each. Gently separate the skin from the flesh with your fingers and insert 7 or 8 rosemary sprigs under the skin of each bird. Slide the sprigs in lengthwise and distribute them evenly all around the breasts and thighs. Rub 1 tablespoon of oil on the skin of each chicken. Tie the legs together with kitchen twine. Salt the skin of the chickens just before placing them breast-sides up in the roasting pan on top of the fennel. Roast until an instant-read thermometer inserted into the thickest part of a thigh (but not touching a bone) registers 165 degrees F, about 1 hour.
Transfer the chickens to a cutting board and let rest for 10 minutes. Carve into thin slices and serve with the fennel.