I don't think they 'live together' in the sense that most of us would mean -- I mean, technically they live at Highgrove and at Clarence House when they're in London, but she's held on to her own home fifteen or so miles away from Highgrove, and spends a lot of time there with her adult children and grandchildren visiting. When you think about it, it's probably a solution more people who marry in late middle age, and have different lifestyles, would choose if they could afford it.
I imagine that kind of thing will probably have to become more formal when Ch comes to the throne, which I imagine C is not wildly enthusiastic about...?