Hi SleepDeprived,
Just for the sake of clarification … I believe that you may have a side to side split mattress where each half of a single mattress (either queen or king size) has different firmness combinations and in this case it would be very unusual to be able to feel the split if the mattress has a suitable tight fitting quilted cover (see post #2 here).
byrum4863 was asking about using two completely separate twin XL mattresses that are put together to make up a king size mattress so that each side can be raised and lowered independently on a king size adjustable bed (which is also two separate twin XL adjustable beds) rather than having a single mattress with different layering on each side.
Phoenix