On a joint PNR, the "status" for all on the joint PNR is the same as the highest elite (so it would have not mattered what the status of the second passenger was) and it could be these secondary factors (each sequence number / check-in order) are also assigned of the highest elite are also assigned to to all on the common PNR. The intent appears to keep the common PNR group together on the waitlist.

If you still have a copy of the BPs you can easily find the sequence number -- it is just below QR on the mobile app (Andriod) and above the bar code on the paper BP