Hi SleepDeprived,
You can read a little more about a side to side split layering (inside a single mattress) in post #2 here. You wouldn’t feel the split itself with an appropriate cover because the foam would be compressed together inside the case but you would feel the transition from one firmness level to another in the center area. it would be more noticeable if the difference between the softness/firmness of each side was larger.
Phoenix