Hi Osahar,
I believe that the adjustable beds they sell are made by Reverie (which also manufactures the Tempurpedic adjustables although they have a slightly different set of features). We have owned a Reverie Deluxe (5D) for over 3 years and have been very happy with it.
There is more about adjustable beds in post #3 here and the adjustable bed thread that it links to. As Mario from Brooklyn Bedding also mentioned … most adjustable beds have options for different leg heights and the Reverie has 2.5", 4", and 6.5" legs that come with the bed and there is an optional 10" version as well.
The adjustable bed itself would be split in a king size (two twin XL’s) but you can use either two twin XL mattresses or a single king on top. The advantage of separate mattresses is that you can adjust each one individually which would be an advantage for couples who prefer to sleep in different positions or where one is sleeping but the other one wants to use the adjustable for watching a movie, reading, etc. The disadvantage would be that the gap in the middle may be more noticeable than having a single king size mattress (although how much you would feel the “gap” in between would depend on the type and design of the mattressand how closely the top surfaces fit together). There are some solutions for turning two twin XL mattresses into a king and some of the pros and cons of each in post #8 here but it really boils down to a lifestyle and preference choice and how important it would be to have each side operate independently vs having a king size mattress and linking both of the twin XL adjustable beds together so they operate as one unit.
Phoenix