Process sweet onion, next 7 ingredients, and 3/4 tsp. salt in a food processor or blender until smooth, stopping to scrape down sides. Set mixture aside.
Remove excess skin from necks and cavities of chickens, if desired. Starting at large cavities, loosen skin from breasts and legs by inserting fingers and gently pushing between skin and meat. (Do not completely detach skin.)
Place chickens in a large roasting pan. Using a bulb baster, squeeze lemon-chipotle mixture evenly into chicken cavities and under skin on breasts and legs.
Sprinkle chickens evenly with remaining 1 1/4 tsp. salt. Tuck wings under, if desired. Position the center of a 3-foot piece of kitchen string under back of one chicken near tail. Wrap string around legs and around body of chicken. Tie securely at neck. Repeat with remaining chickens.
Place chickens, breast sides up, on a rack in a large roasting pan. Bake at 350° for 2 hours to 2 hours and 30 minutes or until a meat thermometer inserted into thighs registers 175°, basting occasionally after 1 hour. Shield with aluminum foil to prevent excess browning, if necessary. Remove chickens from oven; cover loosely with aluminum foil, and let stand 10 minutes or until thermometer registers 180° before slicing.