I don't know about your two mattresses idea, but I do know about something that might help the motion issue. We recently bought a new mattress after 36 years of a water bed (we like the motion of the water!). We finally decided on a latex mattress. There are two kinds of these: one kind is layers of latex or latex shreds between layers of other stuff or a latex topper, and another is this solid latex block about 6" deep. We got the second kind. We find that it is kind on our hips and shoulders, but it's amazing how much we just don't feel the other moving. It also doesn't get divots. The guys at the shop have been in business since 1954 and they haven't had one divot out yet. They also make regular mattresses, so they don't depend on just selling these. In any case, something to think about.