From my experience, the easiest and most effective way to travel from the airport to Petronas would be to take the KLIA express train from KUL to Sentral (about 35 min or so each way). At Sental, transfer to the Putra Line and take it to the KLCC stop which will drop you off right by the towers (10 minutes each way). Many nice restaurants and shopping to keep you busy. Retrace your journey to return to the airport. Vastly less expensive than a taxi and in traffic even faster. Have a marvelous visit.