We talk about it, how much fun she'd have, the extra crap we can carry, etc and, maybe, some day but, she simply loves being on the back and sightseeing and that is the core problem; fear of her attention span, getting distracted and the near non existent margin for error when riding a motorcycle.
On the other side, I love her being on the back and being able to share in seeing stuff together. Her pointing out something over there, me tapping her thigh for something over there. It's just, for us, everything we want in riding, the togetherness, the connection. Sounds sappy, I know but, that's it for us; riding together and we go every chance we get, even to the grocery store.
It takes time to get used to two up, where the gray area starts, to build the feel and instincts and reactions that you have built when it's just you and, frankly, it took away from my enjoyment a good bit, pretty much like starting over. Now, with 1,000's of miles under us on every bit of road I'd go on solo, it is second nature. Maybe not as aggressive all the time but, very relaxed, very 'in' the bike.
And, when I go alone, it brings back that amazing guts y'all remember the first time you ever flogged your Strat!