Hi byrum4863,
As you probably know I think you are looking at a great quality/value choice.
If you are concerned with the crack in between the mattresses themselves then whether you feel the crack in the middle and how much you feel it would depend on the specific design of the mattress and whether the top edges are flush to each other, the body type and sleeping positions of the people sleeping on it, and on how and where you sleep. There are some comments in post #8 here and in this topic that should help you decide on whether the pros and cons of a split king mattress are worth it to you.
A topper would be one of the options (along with some of the options in post #8 here) that can minimize the “feel” of the split between each mattress (if that turns out to be an issue because for some people or some mattresses it wouldn’t be an issue at all) but it would also eliminate the ability to operate each side of the adjustable bed independently because you would risk damaging the topper. If adjusting each side of an adjustable bed individually is important to you then you would also need two twin XL toppers (as well as separate bottom sheets and mattress protectors) as well.
Phoenix