Hi Alius123,
I don’t think there is too much I can add to brass’ excellent comments and suggestions :).
This will happen anyway even with a mattress that uses the same layers on both sides because heavier body types will compress a mattress more than lighter body types. If a lighter half of a couple has softer layers in their side then the depth of compression on both sides may even be more even. As you mentioned though it wouldn’t make much difference if you are each sleeping on your own side although if one of you was sleeping more in the middle you may notice the transition in firmness from one side to another. In some cases if you sleep close together and both of you have a very soft layering then a heavier person can sink in deeply enough that it can create a “valley” and cause the lighter person to “roll” towards them. There is more about split layering in post #2 here.
If you were to choose a 3" layer of a different material to use inside your mattress to replace one of the default layers or to add as a topper then there are some good sources for memory foam, and latex in post #4 here.
There is also a little more in the first part of post #2 here about some of the ways that can be used to accommodate a couple that have very different needs and preferences.
Phoenix