You definitely want a twin XL fitted sheet for each mattress so you can adjust them how you like. I did the same as another commenter and bought a split king set. More on reddit.com

Bought a split Cali king this summer with a L&P adjustable base. Highly recommended for sleeping. Can't be beat. Terrible for everything other than sleeping. Tile floors. I failed to fully appreciate an adjustable split king is literally two separate beds pushed up against each other and nothing anchors the two together. Mind the gap. Def a no-go if you're a frequent spooner. You might have better luck on carpet or a traditional frame. Even on a traditional frame, though, the gap between the two mattresses won't meet perfectly and seal. It will always be there like the cushions on a couch. Good luck! More on reddit.com

If by "split king", you mean two TwinXL beds that you place next to each other without touching, then yes this will work great. But of course you'll have a gap between the beds. If instead you mean two TwinXL mattresses on the same King size bed that either touch or that are enclosed in the same cover/protector, then the benefit will be minimal. Motion still transfers both through the bed frame and through the joint surface between both mattresses. It'll likely improve things a little bit, but I wouldn't hold much hope that it'll make a dramatic difference. More on reddit.com

i mean this is not always an option but my partner and i sleep in different rooms and it's dreamy More on reddit.com