Folders and tags are typically used to classify documents. This classification can then be used when displaying that document. Sometimes a document may want to dictate how it wants to be displayed: for example to show what are the related documents.

The dependent documents may not be aware of the main document that wants to pull them together. However when they are displayed in the context of the requested document they honor that context and show the links coming from the main document.

It may be possible to do this with dedicated tags or folders for that matter. (Something to think further in the design)