onward ticket with confirmed seat: how to get the ticket with Online Check-In only 48h before

Hello, we are flying from Manila to Gunagzhou by China Southern and our flight with Cathay Pacific to Hongkong leaves 50 hours after arrival. Cathay Online Check-In is only 48h before departure time. How should I get the Cathay-ticket with confirmed seat ? China Southern won't do this, isn't it ? Would the official round trip reservation Manila - Guangzhou - Hing Kong with E-Ticket-Number and seat reservation be sufficient ?

Thanks !