We purchased about five years a 12" memory foam king mattress from Lake Mattress NC and the sales person recommended we use the twin adjustable bases. I fear the mattress is giving both me and my wife back pain and I am going to start looking to replace it.
I have always been a bit leery of the twin adjustable frames, although we use them as paired, and wonder if we would be better off with a split king???
Hi grichferg,
Welcome to the Mattress Forum! 
I’m not quite sure what you mean specifically by being “leery” of using twin extra long power foundations under a king mattress. By syncing the units (either with a cable or via IR), the two units will usually stay within 1" or so and move your king mattress up and down in unison, and any Leggett and Platt power foundation would have enough lift capacity to work with a typical 12" memory foam mattress.
Recently there have been a few introductions of king power foundations that are one-piece, but these use only one motor (versus two for your set up). These are seen more as a cost-savings and convenience measure, as opposed to an increase in lift capacity or quality.
Many people do opt for two twin extra long mattresses instead of a single king. The advantage of this is of course the individualized preference of elevation that is allowed on each side independent of the other. This is especially useful when one person wishes to stay elevated to read a book and the other person wishes to go to sleep in a flatter position. There is also the potential for more motion isolation. The downside, and this varies by the type of mattresses used, is that there can be a bit of a small “trough” where the two mattresses meet in the middle. Two twin extra long units can also cost a bit more than a single king mattress.
In the end, I would consider this more of a personal preference in the individualization of comfort as opposed to a better/worse choice.
I hope that information is helpful to you.
Phoenix
Extremely useful, thank you.
Hi grichferg,
I’m glad I could be of assistance!
Phoenix