Rinse and pat dry chickens, inside and out. In a small bowl, mix together the softened butter, garlic and chopped rosemary. With your fingers, gently loosen the skin of the chickens, and rub the garlic butter under the skin. This will keep the bird very moist. Stuff the cavity with the rosemary sprigs and lemon halves.
Prepare the Keg for indirect grilling at 275°F, fill the diffuser kit with water. Place the chickens on the preheated Keg for approximately 4 hours. Once the internal temperature of the meat has reached 170°F, remove from the Keg and wrap in foil.
For an extra flavour boost, toss some rosemary, garlic and wine in the water pan below the chicken, and some flavoured wood chips on the coals.