Thanks for taking the time to share your feedback as well … I appreciate it
I would also make the same comments as I did for dwerzemens about giving the mattress some time for you to adjust to a new sleeping surface and for the mattress to break in and possibly checking some of the “non mattress” suggestions that I linked in my previous reply as well.
If you have a thicker mattress pad (vs a thinner more stretchy mattress protector) then it may be affecting with the feel and response of the foam underneath it and interfering with its ability to contour as well to your body shape which can affect both comfort/pressure relief and support/alignment when you sleep on the mattress. This could result in a “feel” that was both a little softer in terms of surface feel (from the mattress pad) and a little firmer in the layers underneath it that don’t contour to your body shape as well. A thinner more stretchy mattress protector may allow the mattress to perform a little more closely to the way it was designed.
The twin XL size is also 39" wide (twin sizes are sometimes 38" wide and sometimes 39" wide) so two of them together would be a little wider than a king size which is generally 76" x 80".
The gap between two twin XL mattresses that have squared off edges usually won’t be nearly as noticeable with a mattress pad on top but as I mentioned a thicker mattress pad will also have its own tradeoffs in terms of how it affects the mattress. There is more about split king mattresses in post #2 here and the posts it links to.