Complex sentence: He currently tours with Olivia Newton-John as an instrumentalist and vocalist performing John Travolta's part with Newton-John on her No.1 "Grease" hit, "You're The One That I Want", as well as backup vocals.
Split into simpler sentences: He currently tours with Olivia Newton-John as an instrumentalist and vocalist. He performs John Travolta's part with Newton-John on her No.1 hit, "You're The One That I Want" as well as backup vocals.