BBC just wrote that the tickets that were purchased using the stolen passports were booked at the same time.
"The BBC has confirmed that a man falsely using an Italian passport and a man falsely using an Austrian passport purchased tickets at the same time, and were both booked on the same onward flight from Beijing to Europe on Saturday.
Both had purchased their tickets from China Southern Airlines, which shared the flight with Malaysia Airlines, and they had consecutive ticket numbers."
Source:
http://www.bbc.com/news/world-asia-26502843