one-to-one relationship

I have a table that I had split into 2 tables (for various reasons). The
problem is, I've seen a few instances of customers databases where it's not
creating the record in the 2nd of the one-to-one tables, especially when
they had imported their customers from another db, and no data existed for
the fields in the 2nd table.

How do I ensure that, if there is one record in one table with a particular
CustomerID, that it is also created in the 2nd of the one-to-one tables?
This would be for either new customers, or imported customer data.

Many Thanks,

Andy