Check both tag boxes. The default will cause an AND operator. If you change the Options drop-down menu to Show close match results, you should get a result that will be an OR operator for the chosen tags:
Alternatively, click on the Search button and type in the names of your two tags. Press Enter after each tag name. The two tags will now be separated by an AND operator. Click on the + operator between them and you will have the choice to change it to an OR operator.