If you are writing a document with another Mendeley user, the best thing to do to ensure that you both have access to the references you are using is to create a group in Mendeley, invite the other user to join the group, and then insert references from the group in Mendeley Desktop into your document. As you are both members of the group, you will both have access to the references in the group that you use in your document, and will not have to import new references into your library.

If you have two separate documents both containing Mendeley references, copying and pasting one document into the second document will lead to the bibliography for this file containing all the references added to both documents. Hope this helps!