Some airlines insist to give up seats even they are not on the same flight! But on the same day.
Happened to me with BKK-FRA on TG in F
I had seats on both flights dayflight and nightflight ( in one reservation ) ticketet in F. Calling me why I held 2 reservations for the same day, I should give up one of my reservation because both flights are overbooked in all classes.
I not want to discuss about my reason, and said nice and friendly yes " I will give up one reservation until tomorrow"
So I made a new booking waitlisting one of the flights, called TG canceled this leg in the old PNR and after seconds the new PNR with the listed flights was confirmed. Weeks later next call ! Now just asking if we really need this seats ( because no ticketno. in reservation ). I told them ticket is issued and put in ticketno. manually.
SQ has a system that TA`s can not book 2 flights which not fit together in one PNR ! And if you do 2 the same segments in 2 PNRīs it can happen that they cancel both.