Although generally i have travelled solo on this trip i did do four weeks and a fair few miles around turkey, two up, as part of it, without any modifications to the seat at all. I also met a couple in Delhi that were doing a RTW on an Adventure, and they were quite ok with it.
At the end of the day its not the ultimate two up tourer by any means but you can do it and it may well be worth spending some dosh on the appropriate mods on the seat to make it better. But the suspension and cycle parts generally are well up to the job and the engine is easily powerful enough to cope.
Maybe a deciding factor is the size of the people, as a point of interest i am 5' 11/13 stone and my partner was 5'6/9 stone. Hope this helps...
Some day so soon....