Hi mattl0 and welcome to the Mattress Underground 
As you likely know, any mattress you choose will be based on your Stats (height, BMI, sleeping position(s) and any underlying health concerns) and your PPP (posture & alignment, Pressure relief, and Personal preferences).
ILD (indentation load deflection) is a method of determining density by measuring the amount of pressure it takes to compress a 4" square piece of latex rubber by 25%; unlike polyfoam and memory foams, the ILD does directly correlate to density for latex rubber.
You might want to read what @Phoenix has to say about reverse layering in This Thread, progressive layering here as well as their general discussion on layering here. In short, there are instances where a softer layer under a firmer one may be a good sleep solution for sleepers with specific needs…can you let us know what specific solution you are trying to achieve? This will let us make more direct explanations or suggestions!
~ Basilio