Remove and discard giblets and necks from chickens. Rinse chickens with cold water; pat dry. Trim excess fat. Place each chicken, breast side down, on a cutting surface. Cut each chicken in half lengthwise along backbone, cutting to, but not through, other side. Turn chickens over. Starting at neck cavity, loosen skin from breast and drumsticks by inserting fingers, gently pushing between skin and meat.
Combine parsley and next 7 ingredients (parsley through pepper). Rub parsley mixture over breast and thigh of each chicken. Gently press skin to secure. Cut a 1-inch slit in skin at the bottom of each breast half; insert tip of drumstick into slit.
To prepare grill for indirect grilling, place a disposable aluminum foil pan in grill; pour water in pan. Arrange charcoal around pan; heat to medium. Coat grill rack with cooking spray; place rack on grill. Place chickens, breast sides down, on grill rack over foil pan. Cover and grill 1 hour and 15 minutes or until a thermometer registers 180º, turning chicken over halfway during cooking time. Remove chicken from grill; place on a clean cutting surface. Cover with foil; let stand 5 minutes. Discard skin before serving. Garnish with lemon wedges and oregano sprigs, if desired.