A wiki can look more than within itself for the pages that it references with internal links. I encouraged the relocation of conversations tangential to my original wiki. I then imported the page names from these new sites into my own and rendered links to them as appropriate.
Page on another site
An internal link would render as a link to the remote site if it were present there but not on my own. In later versions of wiki I linked to a fabricated pages that explained the departure.
A page would render references to sister pages of the same name when they were known to be present. These references were in the form of site icons at the page bottom or in a margin.
Sister Sites 
mentioned on Wikipedia.
Half Sisters discussed as mechanism for sharing in the presence of complex namespaces.
Coding Tips for Sister Sites when I encourage other wiki implementors to adopt the practice.