Almost anything can be done to make a bike into a 2 up bike. I built up a DR 650 for riding the Trans America Trail 2up. I lengthed the seat base by 70mm and ended up with a seat made by Russell Day Long seats. It is awesome. Suspension and lots of other mods to.
I have closely studied the 690. Its main problem for 2 up is the lack of a subframe, the rear section is the plastic fuel tank whose mounting points aren't good. there is a fix for that but I dont think it would survive long term with the extra weight. With some clever engineering, I reckon a subframe could be made for it.
I had a look at the Husky Terra, the suspension travel isn't much on it, but again, that could be rectified. Its proably not much lighter than your GS800 though.
Will KTM build a neat 800 ?
Here are pics of a 690 seat from the Russell site