1. 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.
2. 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.)
3. 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.
4. 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.
5. Soak wood chips in water for at least 30 minutes. Set aside.
6. Prepare smoker according to manufacturer's directions. Bring internal temperature to 225° to 250°, and maintain temperature for 15 to 20 minutes. Place chickens on upper cooking grate; cover with smoker lid.
7. Cook chickens, maintaining the temperature inside the smoker between 225° and 250°, for 1 hour and 30 minutes. Drain reserved wood chips, and place on coals. Cover with smoker lid; smoke chickens 2 hours to 2 hours and 30 minutes more or until a meat thermometer inserted into thighs registers 175°. Remove chickens from smoker; cover loosely with aluminum foil, and let stand 10 minutes or until thermometer registers 180° before slicing.