There are several methods you can use to combine EndNote libraries.
Import method (NOTE: A YouTube video demonstrating this method is available here):
1) Open the EndNote library that you want to be the final, combined library.
2) Using the menu bar across the top, go to File > Import > File. Then click the "Choose" button to browse for the name of the other library.
3) Select "EndNote Library" from the drop-down menu for "Import Option".
3) Select "Discard Duplicates" from the dropdown menu for "Duplicates".
4) Click "Import."
Click-and-drag method:
1) Open both libraries at the same time. If the libraries fill the entire EndNote software pane, use the "Restore Down" button to make them smaller (without minimizing them completely) and move/resize them so you can see both of them at once. The "Restore Down" button is the one between the "Minimize" and "Close" buttons in the upper right corner of each library's window.
2) Select the citations you want to move from one library to the other (use Ctrl-A to "select all"), then drag and drop the selections into the other library.
Cut-and-paste method:
1) Select the citations you want to move from one library to the other (use Ctrl-A if you want to "select all"), then right-click the mouse and select "Copy".
2) Place the cursor somewhere in the destination library, then right-click and choose "Paste". (NOTE: Using the keyboard shortcuts Ctrl-C and Ctrl-V or using the options under the "Edit" drop-down menu do not always seem to work reliably.)