Any new booking will need to be a new PNR which is not combinable with an existing PNR.
...Mainly for CPU and ...
An elite can only have one companion for CPU. If you have two non-elites on the same PNR with you, no one -- including you will be eligible for CPU.
E+ can be added for a non-elite on a separate PNR as mentioned before (note only Plat and above can add more than 1 E+ companion).
Premium security / boarding does not require the same PNR.
Extending your bag elite waiver to a companion may be the benefit you need to consider if a common PNR is really needed.