MultiGUI is a simple wrapper-type thing to parent two child windows/editors onto the main window passed to your plugin by the host. A screenshot:
That is two separate VSTGUI editors, contained within the same window.
Basically, MultiGUI consists of two files: MultiGUIEditor.h and the corresponding source file. MultiGUI itself is a single class which inherits from AEffEditor, and passes all the setParameter etc. messages onto it's child editors. You use it by inheriting from the MultiGUI class, and calling the addEditor() method to add editors to it. You can have as many child editors as you like. The download includes the MultiGUI files in an example project (seen above).
Note: MultiGUI does not work correctly in Tracktion on OS X. Tracktion seems to handle windows differently from every other host on OS X, so if you load a plugin with a MultiGUI editor, the title bar for it is not displayed, and there is no way to hide the gui, or move it, without deleting/removing the plugin from the project. There may be a way to fix this in the future, but I'm kind of at a loss myself. Every other host tested (including Tracktion on Windows) works fine.
Download MultiGUI v1.02 (298kB)
Of course, MultiGUI isn't particularly useful if you just use it to add two VSTGUI editors to your window - the main purpose of it is to allow for a combination of a VSTGUI 2d interface with a 3d opengl one, courtesy of VSTGL. A screenshot:
This screenshot consists of a standard VSTGUI interface on the left, and an opengl one containing a spinning pyramid on the right. You can download the source for it here:
Download MultiGL example v1.02 (313kB)
You can see a more advanced example of a MultiGUI/VSTGL interface in Harmonical.