One of the Unit Key 3 or Key 3 Delegates can connect the parent’s account to the scout via the scout’s profile in SB+. I posted some graphics and an exlanation in this thread:
https://discussions.scouting.org/t/second-parent-link-in-scoutbook/509564/2
If the parent relationship already exists under one of the other Member IDs, the council registrar needs to move the parent relationship to the “correct” (current) Member ID. Using Manage Member ID unfortunately doesn’t accomplish that. I’m not 100% sure what would happen if the relationship already exists for one of the “secondary” Member IDs and you add one to the primary Member ID.