It's primary purpose is that we each ride our own -- hers is not any better at carrying passengers than mine is. I don't mind carrying a passenger, and the Monster doesn't mind carrying a passenger, but the passenger (or at least certain parts thereof) does mind, after a bit.
We have, at times, when one or the other of our bikes is somewhere else, ridden either hers or mine two-up, but the limit on that is not much more than 100 miles. Each on our own, we can do 200 or 300 miles comfortably. Me on my own, I can do 500 to 600 in a day pretty easily, and up to 900 if I need to, but she is not as experienced in long-distance riding (and doesn't aspire to be).
I'm looking at sport-touring bikes that could do well for longer trips two-up. I've only tested about half of my list of contenders, but the current leader in the reasonably-priced category is the Moto-Guzzi Norge. (In the unlimited category, I'd already have a MOTUS in the garage, and not be looking any more.)
PhilB