I have a Kinetic Magnum that is built for 2 people...it has a big seat as well as foot rests for the passenger. It also has a variator, so it will go uphill with 2 people on it. I think the weight limit is something like 350 lbs. or so.
I would not take 2 adults on any moped that is not rated for 2 people. The tires have to be rated to take the weight.