At the place I was at last week a guy pulled in Thursday with a 5th wheel and a toad on the back of that. I didn't even think it was legal to do that but then someone told me different states have different laws about chaining towables so, meh...
The issue for me was it was a really long rig. I could not imagine negotiating any smaller roads, towns with that thing and heaven forbid trying to back it or turn it around - route planning would be crucial.
Other than it looking ugly, not being an elegant solution to towing what looks like a pretty small 5th wheel and needing a drop hitch on the tow vehicle to level the first trailer out, I don't see that rig as being unsafe or anything. In fact if one could make a purpose built "5th wheel adapter" I bet they'd sell a few of them...