Multi-level searches are possible using the options in the lower part of the search form.
When you do a second search and you tick the option Extend you will see the results of both the first and the second search. For instance: I click New selection and choose 'Sample Inc. in the field 'Client'. Then I click the Search button. After the results have been displayed I go back to the dialog box, I click Sample Holding and in addition I click Extend. Then I click the Search button. This two-level search yields all documents that belong to Sample Inc. AND Sample Holding.