The two longest trips I ever went on were both two up with a mate on his Guzzi, sharing the riding. One was from Melbourne to Adelaide and back, around 1,500 klm, and the other was from the Sunshine Coast to Cairns and back, around 3,200 klm. There's a ton of bikes I could name that would make those trips a horror story two up, but the Guzzi was a great tourer with a nice long comfortable seat and quite low from top of seat height to the ground. I could imagine it as the sort of trip where a BMW would be good as well.