Um, I'm pretty sure there are two seat ultralight airplanes - at least here in the Czech Republic. And IIRC that's also the maximum number of people that can fly on an ultralight here.
Yes the categorization in Europe seems to differ a lot. I flew in a Czech ultralight once, it was a Lambada, I forget the manufacturer. Very nice aircraft by the way. Just very very cramped with its 2 abreast seating.
