Driving with the jetski on the lift could decapitate someone who crashes the rear of the van. In addition it might block rear lights, depends how they are set up.At least two reasons I can think about right now why it would not be a good idea.
You should take a look at US toy haulers and if you could replicate the functionality in a van. For some reason they haven’t gained traction here in EU.
Really the smartest move in my opinion would be a shorter van and a trailer that can take both the quad and jetski sideways so it does not have to be so long. As a plus side you don’t have to smell engine oil and gasoline while sleeping.
Show this video to your favorite welder 
It shouldn’t take too long to figure out how to gut and rebuild some old trailer to allow side loading. Make sure the trailer uses same wheels as your van, so you are always carrying at least two spares.
Also a two story option…
This is a neat idea.