Hi katroushka,
It would depend on the the type of mattress and the materials and components inside it (including whether it has any edge support), the firmness of the support core, the thickness/softness of the comfort layers, your body weight and sleeping positions, and on where you sleep on the mattress over the course of the night (for example it would be more noticeable on a softer mattress without specific edge support if you sleep in a spoon position in the very middle of the mattress).
There are also some comments in post #8 here and in this topic about the pros and cons of a split king mattress (two twin XL mattresses instead of an eastern king size mattress) that may be helpful as well.
Having said that … the only way to know for certain whether a specific split king mattress would work for your specific body types and sleeping styles would be based on your own personal experience so the return policy would become a more important part of the value of any online purchase where you aren’t confident that it would be a good “match” for you in terms of all your individual criteria (including center support) that are important parts of your personal value equation.
Hopefully some of the members here that own a split king BME in the firmness levels you are considering will see your post and share their comments but they may have different body types or sleeping styles than you do and their experience may be different from your own so when you can’t test a mattress in person then the most reliable source of guidance is always a more detailed phone conversation with a knowledgeable and experienced retailer or manufacturer that has your best interests at heart and who can help “talk you through” the specifics of their mattresses and the properties and “feel” of the materials they are using (fast or slow response, resilience, firmness etc) and the options they have available that may be the best “match” for you based on the information you provide them, any local testing you have done or mattresses you have slept on and liked or other mattresses you are considering that they are familiar with, and the “averages” of other customers that are similar to you. They will know more about “matching” their specific mattress designs and firmness levels to different body types, sleeping positions, and preferences (or to other mattresses that they are familiar with) than anyone else.
Phoenix