In an 8- to 10-qt. pot, heat 1 qt. water, the salt, and sugar over high heat, stirring, until salt and sugar dissolve. Remove from heat. Squeeze lemon juice into pot, discarding seeds. Add lemon shells, peppercorns, bay leaves, half the onion, and the garlic. Stir in 5 cups ice water. Let cool to room temperature.
Put chickens in pot and set 1 or 2 small plates on top to keep them submerged; if needed, add a little more water so chickens are covered. Chill at least 4 hours and as long as 24 hours.
Preheat oven to 400°. Drain chickens, discarding seasonings. Rinse chickens and pat dry inside and out.
Set a large cooling rack in a roasting pan and set chickens breast side up on top. Stuff large cavities of each chicken using remaining onion, the carrot, celery, 4 sprigs thyme, and 2 sprigs each sage and rosemary. Crisscross ends of legs, tucking cavity closed, and tie with kitchen twine. Wrap a long piece of kitchen twine crosswise under each chicken at wings and tie to hold wings snug. Make 2 bundles of remaining herbs; tuck under twine at wings, draped over breasts. Rub chickens and herbs with oil.
Roast chickens until a meat thermometer inserted deep in thigh next to bone reaches 170°, about 1 1/4 hours, draping tips of legs with foil if they start to get dark. Tip chickens to release juices and transfer chickens to a carving board (save pan juices). Tent loosely with foil and let rest 20 minutes.
Tip juices from board into roasting pan. Strain pan juices, skim and discard fat, and pour into a small pitcher. Remove twine from chickens and set crisp herbs aside. Cut chickens into pieces, using kitchen shears and a knife (save backs for broth or other uses). Arrange chicken, vegetables from cavities (they'll be crisp), and crisp herbs on a large platter and serve with pan juices.