Thanks Mark. I think my basic design was flawed too. I moved the FK in the Accounts Table. The Account belongs to the customer. Not the other way around. So..it seems this design works better (and no fuss from SQL!):
Old Design:
CUSTOMERS
------
CustomerID (PK)
AccountNumID (FK)
FullName
Address
ACCOUNTS
------
AccountID
AccountNum
NEW Design:
CUSTOMERS
-------------
CustomerID
Name
Address....etc...
ACCOUNTS
-----------
AccountID
CustomerID (FK)
AccountNum
...etc...