Updating two identical formatted tables with one form

At present I have a membership database with all members held in one table. I
have been asked to split this table into two tables - family and individual
members. The user still wants to view all the records in both tables using
the same form as at present. However if I use a UNION query on the two tables
and link the form to this query, I have found that although I can view all
the records from both tables, I now cannot update them.
Any advice on a better way to design and link the tables would be appreciated.