You can insert the main content of another document or publication into layouts by using an NPSOBJ instruction. In your code replace file-path with the complete path to the file to be inserted:
<npsobj includetext="file-path"/>
The inserted file will be completely managed by CMS as a reference. A link will be created between the two files that will be managed bi-directionally and can be edited exactly like other internal links.
In the Contnet Navigator, this type of link is displayed in the Content section.