Heat grill for indirect grilling: Light one side of grill, leaving other side unlit. Close lid; heat grill to 350F.
Trim and discard excess fat from chickens. Squeeze lemon juice into small bowl; reserve lemon halves.
Add barbecue sauce and seasoning to juice; mix well.
Place 2 each lemon halves and garlic cloves inside each chicken cavity.
Place chickens, breast-sides up, on grate over unlit area; cover. Grill 1-1/2 to 2 hours or until chicken is done (165F), turning and brushing occasionally with sauce for the last 20 min.
Remove and discard lemon halves and garlic from chickens before serving.