Hi willwill,
Post #8 here has some options that I’m aware of (for the mattress) but I don’t think there is a perfect solution to turn a split king mattress into a single king size.
If the issue is that the adjustable beds are separating (rather than the mattress) then some rubber caster cups (such as these or something similar) under the legs or a strap around the legs that can be cinched very tight may help prevent them from sliding apart.
I would keep in mind that with all of the solutions that will fill in the gap in the mattress you would lose the ability to raise and lower each side independently (at least without first removing and replacing the material in the middle) which is the main reason for a split king in the first place.
Phoenix