We use two Twin XL mattresses strapped together to make one (effective) King size bed. This gives us the option to pick a firmness that we each prefer without having to compromise. We use 1.5" spacers to bring the top of both mattresses to matching height.
https://www.mattressfirm.com/mattres...imensions.html
We use a mattress strap and gap filler (as shown below):
https://www.amazon.com/EPHEDORA-Brid.../dp/B07KXXTDLR
Once the bed is made, it is nearly impossible to tell that the bed is made up of two side-by-side mattresses.