Remove and discard giblets and necks from chickens. Rinse chickens with cold water; pat dry. Trim excess fat. Place chickens on a cutting surface. Split chickens in half lengthwise. Starting at neck cavity, loosen skin from breasts and drumsticks by inserting fingers, gently pushing between skin and meat.
Combine rosemary and remaining ingredients except cooking spray. Rub mixture under loosened skin and over breasts and drumsticks. Place chicken halves in large zip-top plastic bags; seal and marinate in refrigerator 4 hours or up to 2 days.
Preheat oven to 500°.
Remove chicken halves from bags. Place chicken halves, skin sides up, on a broiler pan coated with cooking spray. Bake at 500° for 30 minutes or until a thermometer inserted into meaty part of thigh registers 175°. Remove chicken halves from pan; cover and let stand 15 minutes. Discard skin.