In an 8- to 10-qt. pot, dissolve the salt and sugar.
Put chicken in pot and set 1 or 2 small plates on top to keep submerged; if needed, and a little more water so chicken is covered. Chill at least 4 hours and as long as 24 hours (if thawing frozen bird).
Preheat oven to 425°. Drain chickens. Rinse chickens and pat dry inside and out.
Set a large cooling rack in a roasting pan and set chicken breast side up on top. Stuff large cavities of each chicken using onion, the carrot, celery, 4 sprigs thyme, and 2 sprigs each sage and rosemary. Criss-cross 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. Turn oven down to 350 after about 45 min - 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 and save; we will use for gravy or pot pie or stock. Remove twine from chickens and set crisp herbs aside. Carve and save ALL the bones, skin, cartilage you name it anything you are not eating that will go into the stock
tomorrow immune boosting STOCK ... and then soup, pot pie and easy chicken salad. :)