Place butter, herbs and salt in small bowl, mash with fork until blended to a thick, granular paste.

Starting at neck of each chicken, carefully run fingers under skin of breasts to loosen. Starting at each side of main cavity, carefully run fingers under skin of thighs to loosen. Rub herb-seasoned butter on chicken under loosened skin and on outside of each chicken. Truss chickens by tying legs together and tying wings flush to sides. Enclose chickens in extra-large resealable plastic bags. Chill at least 6 hours.

Remove spit from rotisserie attachment and prepare barbecue (medium-high heat). Thread chickens on spit, pushing close to center. Reattach spit to motor. Place shallow disposable foil pans under chicken for drips. Close grill, inserting stem of instant read thermometer through hole in roof of grill to monitor temperature. Maintain temperature at around 400 degrees by opening vents to increase heat and closing vents to reduce heat. Cook chickens until thermometer inserted in thickest part of thigh registers 170 degrees, about 50 minutes. Turn off grill, but do not open. Let chickens rest in covered grill for 10 minutes.

Slide chickens off spit and onto platter. Cut each chicken into 6-8 pieces and arrange on platter.