Hi steelwindmachine,
I’m glad you already found some good information on the site to answer your questions.
In addition:
The main advantage of a side to side split is that you can choose a different initial design and firmness level with different levels of comfort and/or support for each side of the mattress which can certainly be beneficial for couples that are in different weight ranges or that have different needs and preferences in a mattress. Having a side to side split also allows you to “fine tune” the comfort and/or support on each side of the mattress separately from the other side by rearranging layers or replacing a layer on one side of the mattress without affecting the other side. There is more about the pros and cons of a split layer mattress in post #2 here.
Another advantage many people enjoy with split layers is the ease of assembly (smaller pieces), and the ability down the road to customize only one side if one person’s preference changes, even if you start out with the same layers.
Thanks for searching the forum and I’m glad you found your answer.
Phoenix