In a small bowl, mix 2 tablespoons olive oil, lemon juice, thyme, rosemary leaves, sage, and 1/2 teaspoon pepper.
Rinse chickens and pat dry. Pull off and discard lumps of fat. Rub olive oil mixture all over chickens and set birds breast up and at least 1 inch apart on a rack in a 12- by 17-inch roasting pan. Rub remaining 1 tablespoon olive oil all over garlic. Place garlic heads on rack beside chickens.
Roast in a 425° oven until garlic is soft when pressed, about 1 hour, and chickens are well browned and meat is no longer pink at thigh bone (cut to test), 1 to 1 1/4 hours; if garlic is done before chickens, transfer garlic to a dish and keep warm. Tip juices out of body cavities into roasting pan.
Put chickens on a large platter, surround with garlic heads, and garnish with rosemary sprigs. Keeping chickens warm, let rest 5 to 10 minutes.
Carve chickens. Separate garlic cloves and squeeze garlic from peels to eat with chicken. Add salt and pepper to taste.