There are often times where a project wants to share some issues or documents with one or more other projects. Today, we deal with this by either branching the document, copying the issue, or creating an "integration project" which is a new project that shares the security groups of the original project. Each of these addresses the need in some way, but they all fall short in one way or another.
For instance, the "integration project" allows two or more project groups to collaborate on a set of issues/documents, however, it also clutters the project filter and forces users to track many different projects under the same umbrella.
A more ideal solution would be to allow an issue or document to be shared with another project such that people from either project can view it and those with appropriate roles from either can edit it.