My guess is, you're talking more of a scooter. I've seen some moped seats that were kind of long, and you could squeeze 2 people on them. BUT, as far as two average size adults, I don't think there are too many that would work well.
I saw a big (big for 50cc) Kymko scooter, it had lots of room for two and seemed to pull up hills well.
There may be some, but I'm not aware of any. Someone with more experience might be of more help.
This may not help. have you gone to www. "dogpile" "Infoseek" "Altavista" or "Yahoo" (or even Mamma) and asked about 2-seaters? It might produce something.