Travel initiary two separate tickets

Hi

I am travelling to Shanghai for work and are planning to stay 1,5 week with a stop over in Japan for the weekend.
However I am considering booking it as two individual trips:
Ticket 1: Cph - Shanghai return ticket (6th - 16th of May)
Ticket 2: Shanghai - Tokyo return ticket (10th - 12th of May)
All will be booked before so I should be able to apply for two 144h transition sessions each time I arrive to Shanghai I understand.
Just want to check that it is not mandatory that it is the same ticket/booking reference?
Also I used the 144 h transit mid april also, is there any limitations to how often it can be used?

Many thanks