What will work depends on your security settings...
If the document already exists within another document repository, then
create a link to it from each workspace. This assumes all users have rights
to the repository.
If the document is loaded into the 1st workspace, the create a link to it
from the 2nd workspace. Note that this assumes the users on the 2nd project
have the rights to view the 1st project's workspace.
If not, load it into both. I'm assuming the documents are useful for
reading/information rather than as project artifacts.