You'd need to build a query that provides the needed data, then base your
form on that query. If you open the form in
Design view and click the Build button to the right of the Recordsource
box, Access will launch the query designe, and
you can build your query from there (note: this won't store the query
where you can edit it, so if you need it for other
things you might want to build your query first, then base your form on it after).
Note however that basing a form on a query can cause some unwanted results
IF your query isn't updateable so take care
when building the query. In my experience, most forms are based on a
single table with Subforms used to show Child