It was a mistake to not have 1:1 relationships from the start.

With a 1:1 relationship, you can better confine rows in 2 different tables to each other. Also when using advanced find or relating items in workflows from either side of the relationship it works out much better than having to design the "dirty 1:1" hack of setting up 2 1:N relationships on each side and attempting to force each side of the relationship to update the other. Orphan records can still come out of this dirty 1:1. If native 1:1 relationships existed, querying on each side of the relationship would be a joy.

(Also if 1:1 relationships were used between the 2 tables it would make for a cleaner Internal N:N relationship in the non-customer facing internal D365 intersect table)