It was at the Grateful Dead shows in Las Vegas, summer 1994, this show accually, where I first witnessed a modified school bus. I loooked at it and thought "thats kinna weird". It had a high roof and a caboose style platform on the back. I stood looking at it for a while, then it dawned on me: "those are VW busses on the roof"!! I was hooked.
Over the years, I've wondered if the VW(s) are for added light and head room, or is it accually an upstairs? I've since seen that both can be done (sometimes on the same vehicle)! I've collected many pictures from the internet of different examples. Some with the VW facing fowards, others backwards. Some with it centered, others foward or aft. Many possibilities! Some varients I've seen are: The Bug Bus, A VW bus with bug on the roof; a Citroen van with a 2cv on it, etc..
The next step was to wonder how it's attached. I wondered If the "parasite" vehicle was just welded to its "host", skin to skin, or was there maybe brackets, braces or other supports? I'm still not sure as to this, though the best example I've seen was welded on, then had fiber glass and bondo applied to the seam.
Evolution:
The earliest example of this style of raising the roof that I've seen may be from the book "Roll Your Own". I have no idea who "invented" this or when, but I have a theory as to how.
Please compare the following pictures:


I think the first person to do his was inspired by the California Zephyr observation cars of the 50s. Then I read this which seems to agree: http://en.wikipedia.org/wiki/Vista-Dome
This is an ongoing study, and any input is appreciated. Eventually, an experiment will hopefully be attempted.