Hi tavak,
Sorry I should have read your post more carefully. Two twin XL’s are the same as an Eastern King (which is 76" x 80") but a Cal King is 72" x 84" so they wouldn’t be long enough and would also be too wide.
Some of the better options and possibilities in the area are listed in post #2 here but I would call them first to make sure they can provide you with the type and ILD of any latex in the top layer of their mattresses so you have a reference point. For the PLB mattresses you can see the ILD specs here.
I would keep in mind that this will give you a general idea but it’s not only the top layer of a mattress that you will feel and the layers below it will also affect it’s feel and performance to different degrees but it may give you some useful ILD reference points.
Phoenix