Originally Posted by MJS
How big is your passenger? Long legs or short?
I've done Canada and back (mostly paved), lots of Baja (mostly dirt) and Panama (fair amount of dirt) two-up on a 1200GS. The bike was fine for that. Also did Alaska on a 650 Dakar. That sucked two-up.
It is really about your passenger being comfortable. If they're not, you won't be either.
I've done all of USA and central America (plus Colombia/Ecuador) two-up on a DL650. It's more than sufficient.
MJS has it right- passenger comfort is key.