The simple answer is: yes, if you base the form on a query, then updating
the form will update the tables underlying the query. The caveat is, when
joining multiple tables it is easy for a query to become "nonupdatable",
which means basically it is read-only. A simple join between two tables on a
single key field should be updatable, but it won't necessarily be. The
easiest way to check it is to create the query and then run it. In the
resulting window, see if you can update the fields. If you can, the query is
updatable and you could use it for the RecordSource of the form, no VBA
required (unless you want to do things like error checking before
inserting/updating records, for example).

If it isn't updatable, what I would recommend is that you create two
queries, one for each table. Base the form on one of the queries, and use
the other query to create a subform on the first form (you can easily do
this by simply dragging and dropping the query onto the form). I would
assume that the two tables are related somehow through some key field, so
you would specify that field as the link between the parent form and the
child form (subform). Access will take care of updating the underlying
tables for you, no VBA required (same exceptions as noted above).

Carl Rapson