You can use an EndNote library for multiple Word Documents.
However, refrain using multiple libraries on a single Word Document to avoid linking problems.
When you send your word document to other people, we cannot assume that they have or use EndNote software. Therefore, it is a good practice to convert all the citations to plain text before sending it to other people:
Remove Field Codes > Save the Document
There are a couple of ways to insert citations into your text:
Find and insert citation(s) to the document (from Word)
(in Word) Position cursor > Find Citation(s) > Search > Highlight reference(s) > Insert
Find and insert citation(s) to the document (from EndNote)
(in Word) Position cursor
(in EndNote) Highlight reference(s) > Click Insert Citation button
How to edit or remove in-text citation?
(in MS Word) Highlight citation > Edit & Manage Citations
How to change the style of references?
(in MS Word) Format Bibliography > Pull down the Output Style Menu