Hi Bcgirl,
Either way works well but I would personally tend to use thicker layers of wool as a topper because you can choose the thickness of the wool topper, air it out in the sun (wool is self cleansing if it’s put in the sun) more easily than a cover, and you can also replace or change the wool topper if you need to without replacing the entire mattress. It provides more flexibility in other words than a thicker layer of wool quilted in a cover. Thinner layers of wool in a cover can be a great option to help with sleeping temperature and act as a fire retardant without having to buy a thicker topper so it would really depend on how much wool you preferred to sleep on.
There are several methods that can help a mattress accommodate a couple with different needs and preferences (see the first part of post #2 here) and a split layer construction is certainly one of them that can be a good option. Snugsleep makes some high quality mattresses with very flexible designs but I would also be aware that there are other similar “choose your own layer” mattresses that use the same materials that can also be split that are also available (including in the list of online retailers and manufacturers I listed in my earlier reply that ship throughout Canada) that are significantly less so I would make some careful value comparisons before purchasing a mattress in that price range to make sure that the higher cost is the best match for your personal value equation. I would also make sure you do some careful and objective mattress testing for PPP together because some designs may be a good match for both of you without needing split layering.
Phoenix