If I bought a 10 acre parcel, and the building code was a 1 acre minimum to build a house, I could subdivide the land into 10 lots and build 10 houses.
If I subdivided into 10 lots for the express purpose of building 10 houses, and the code then changed to a 2 acre minimum before I built all 10, I should be grandfathered and allowed to build 10 houses.
If the code changed to 2 acres before I subdivided, I can only build 5 houses.
If I bought two separate lots, in separate transactions, they are two lots and can each be sold or built on...UNLESS the jurisdiction has a rule/law that says they are treated as 1 lot if owned by the same person. In that case, as it seems to be in this case, I think the proper way to avoid the problem would have been to will each lot to a different sibling.