looking at the trailer and chris's comment about lift, I used to work for a livestock haulier who used parkhouse bodies on the trucks, the upper deck was lifted by cables attached to a hydraulic ram, so my thought is why with all the various engineering skills that we have on here could it not be possible to convert such a thing !!
As it stands, that trailer has two centreline pillars which pass through the top deck and support the roof. The top deck is welded to those supports, creating a rigid structure. There are ways of getting a car onto the top deck, but as Aaron has observed you could never work on it up there

There are different types of double deck trailers which have a floating floor, but they have limitations to the weight of the top deck and also restricted width due to the lift mechanism

The taillift that Aaron linked to is the kind of thing I had in mind, with the central third removed, allowing easy and height adjustable access to the chassis

All food for thought though