If you wish, you can determine which photos are contained in common between two collections (perhaps you want to remove duplicates?) or you can merge two collections together (the union). To do so, open the first collection, then shift click the second. Now, if there are pictures in common, they will be displayed in the workspace. In the upper left hand of the workspace, a Venn diagram symbol will appear with the intersection area shaded. Click on the symbol and the union of the two collections will come into view and the symbol will change to reflect that a union is displayed. Click again, and the intersection is shown. You even can apply these two functions to more than two collections.