Hi E.J.M.
Radium manufactures Talalay latex in Holland which is widely available in Europe and both their blended Talalay (which is 70% synthetic latex and 30% natural latex) and their 100% natural latex are very high quality materials. There is more about the different types and blends of latex in post #6 here.
If you are referring to a twin XL mattress that is 100" thick (although I have no idea where you would find something that thick) then adding an additional 5" on the bottom would be unlikely to make any difference for most people but adding an additional 5" on top that has a different firmness level would probably be noticeable for most people depending on how much firmer or softer it was than the 100" below it. If it was the same firmness then they probably wouldn’t notice much if any difference.
This would be very subjective and there wouldn’t be a consistent answer between different people because it would depend on the specifics of all the layers, the weight and body type of the person and on where they were in the “princess and the pea” to the “I can sleep on anything” range of sensitivity.
In a “typical” mattress that was say 6" or thicker then adding a firmer layer on the bottom would make less difference than adding a softer layer on the bottom which would would make “some” difference for “some” people because it would compress more and affect how much they were sinking down into the mattress.
Adding an additional inch on top that was either firmer or softer would make a bigger difference that most people would feel.
If the additional layer was the same firmness as the mattress then the additional thickness would be softer but whether someone was able to feel a difference that small would depend entirely on the person and some people would feel the additional thickness and for others it may not be noticeable at all. There is more about the effect of thickness in post #14 here but these are such subjective questions and there are so many “it depends” in the answers that it’s not possible to predict a specific answer that would apply to a specific person and it’s only possible to identify the “direction” of the change.
Phoenix