In EndNote it is easy to copy references from one library to another.
For this you open two libraries via File – Open.
The best thing to do is to show both libraries below each other via Window - Show all windows stacked.
Your screen looks like this:
Now you can select references in one library (click on it, to select more click with the use of the Shift- or CTRL-key).
Now you can drag them to the other library.
Via File – Import – File you can also copy a library into another. First you open the library in which you want to save the data.
At Import Option you choose “Endnote library” and at Import File you state the EndNote library from which you want to copy the data. Linked PDFs are also copied.