Double entry. I am from Greece.

Hi,
I will arrive to PEK airport on 26 September with flight from Istanbul through Kazakhstan. On 28 I depart for North Korea and Pyongyang with Air Koryo. The Koryo tours in Beijing told me when I ask that he can send my tickets to Pyongyang by email to show during my arrival on PEK to have the 144 hours free entry visa. On 03 October I return from Pyongyang to Beijing and in 15 hours the next day on 4 October I fly back home again with Astana air. My problem is that I have my voucher of my flight but I can't have the ticket and the seat before my entry in China. What do you suggest to me for the 2 entries.
Thank you