If you install this plug-in module, you'll find a new sample in the New Project wizard (look around in the "Samples" folder). So, you can open this sample and look at the sources (and learn from them and build your own multiview editor on top of it).
When you install the sample (in the current development IDE) and then expand the top package, you'll find a file called "abc.story". This is an example file. All files with the extension ".story" are handled by the loader that the "MultiView API Sample" plug-in sample provides. Here's the icon provided by the loader:
When you open the above file, you see this content in the XML view:
And, when you click the Design toggle button at the top of the Source Editor, you'll see this:
(By the way, the content that you see in the sample "abc.story" file is, coincidentally, about creating NetBeans plug-in modules...)
And, this implementation of multiview support uses a newer multiview support API, one that is made specifically for XML files. As with several other APIs, though, this API has not been finalized, so beware of assuming otherwise.