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.
Sorry, I was referring to the same legal definition for CFR Part 103 ultralights as in the link.
So how is flight training handled for those I wonder?
You can still go to flight school first, if you want, and get training in a dissimilar two place light sport aircraft, a Cessna, etc. There are also places that do ground school aimed at ultralights with no in-air training, watch youtube videos, or just go for it.
There are two-place similar aircraft that you can train in. They're just not classified as ultralights.