One of the disadvantages of Omeka is that an item can only be linked to a single collection. A simple way of overcoming this is to use a technique combining the plugins Exhibits, Simple Pages and tags. 

To set this up add a new Exhibit as you would normally and then add a page. In the page add the following Simple Pages shortcode [items tags="tag"] .

As tags are added more items will dynamically be displayed within the exhibit page.