I don't remember, I saw it when I was eleven. But the attached portion sits closer to the horse's hindquarters(obviously). I don't think it's any more uncomfortable than two people riding double, other than it's in saddle form. From what I can see int he picture I have (which isn't a lot) it might be attached to the pad, or strapped on in some way to the saddle.
Hm... I imagine the kid sitting back there is probably about as heavy as full saddle bags, and his feet would be so short there is no way to kick the horse in the kidneys, so... I guess it isn't inhumane, but it seems a little silly to me.