You can combine your previous searches in PubMed by viewing your Search History. Click the Advanced Search link above the search bar to find this option.
The searches you have run this session will be displayed under Search History. The number on the left is the search number.
To combine searches, click "Add" next to the search(es) under History. Searches are added to the Builder and to the search box at the top of the page. You may also enter new terms to the Builder. Combine the searches with AND, OR or NOT in the dropdown menu. Edit the search box at the top of the page if you like. Click Search to run the new search or Add to history.
Note: Search History will be lost after eight hours of inactivity.