Hi Turbo man
My experience is that anything is possible and people search for too many solutions before they set off
My girlfriend and I rode 2 up on an XTZ750 Super Tenere from the UK to Cape Town in 1995 just before the dawn of the Internet and we knew zilch before we set off, absolutely squat. In all honestly we were dangerously ill prepared but what the hell, we got to our intended destination after 12 months and had fun on the way.
The bottom line is that no, riding any motorbike, even a 125cc bike 2 up on sand is nigh on impossible. We tried it for 100 yards and gave up, turned around and thought about going home to buy a Land Rover. I remember the tear stained pillow (

) in the tent as we discussed our options. It is just so difficult for a rider of any ability, and the risk of injury to the passenger who hasn't got a feeling for what's going on (does the rider in sand ???), well it would have killed our relationship if we'd done another yard I reckon.
But in the campsite a few days later we met 2 guys, one of whom happened to be ex Royal Navy as I was, we all got on like a house on fire, and they took my girlfriend and the luggage in the Landy. We didn't like being a burden to anybody so quite often I helped out by going on ahead on the bike to look for good routes through the dunes and so on, and my girlfriend cooked great meals in the evening and we bought them tons of cheap African

and things turned out just fine. Our paths continued to cross actually quite often until we finally split up when they departed in Benin as they gave up and flew home
Me and then girlfriend then did Central African mud 2 up and that is just as much fun as sand, if not more

Plenty of days at walking pace, me off the bike pushing, her walking serenly alongside, gently glowing in the heat
But if you want to ride 2 up, where there's a will there's a way to get to your destination. Stick the bike on a train/truck/canoe or bum lifts off locals/overlanders, it's all good fun and only adds to the experience. In fact we did all 5 of those things I just mentioned, happy days