Dear MGAlbion,
Look no further than this
delightful thread from Pfaelzer about a two-up ride through Fujian.
Now, there is a caveat: Pfaelzer's Jialing is easily
the most modified JH600 on the planet, with a better shock and better wheels in particularly, so your mileage may vary. And I speak with almost no experience riding two-up.
But Pfaelzer and I are both in the 90-100kg range (more than a tad more than the lithe Milton and welterweight Motokai) with the same panniers, and I've ridden thousands of kilometers under load. The bike shows tremendous heart. She just gives and gives and gives.
Hope that's useful!
cheers