After conducting multiple searches in a single session, you have the option of combining them.  If you log out and return later, you will not see the searches you performed in the previous session so you will not have the option of combining them.

1.  Conduct 2 or more searches.

2.  Click the Recent Searches link. 

User-added image
If you do not see this link, it means you have not conducted any searches in the current session. 

3.  You will see a table containing the searches you have run during the current session.  Each line is numbered and provides search details such as limiters, databases searched, and number of results:

User-added image
4.  Take note of the numbers you want to combine.  Enter them into the search bar provided, including the Boolean operator AND between them.  Click Search:

User-added image

5.  The combined search you just conducted will now appear in the Recent Searches section, which will allow you to combine it with other searches if desired:

User-added image