Hi bedhead,
Outside of dn’s great comments and suggestions … the first part of post #2 here has more about choosing a mattress that can match the needs and preferences of a couple with very different body types, sleeping styles, or preferences.
There is also a link in the tutorial post to the members here that sell mattresses online and many of these sell mattresses that can be customized separately on each side.
Some of the better options and possibilities I’m aware in in the general Los Angeles region are listed in post #2 here.
Phoenix