Hi mattressseeker,
There would be little if any practical difference between two 3" support layers and a single 6" support layer if they were all the same type and blend of latex and the same ILD and were inside a tight fitting cover with 3" inches of latex of the same ILD on top of them. Two 3" layers would respond a little more independently and because the elasticity of the top 3" wouldn’t be connected and “pulling back” on the bottom 3" when it compresses and “in theory” it may act a little bit softer but in practical and real life terms most people wouldn’t notice any difference in terms of performance or firmness. If a single 6" core with a 3" comfort layer was a good match for you in terms of PPP then there would be little benefit in having multiple 3" layers that were the equivalent ILD.
Also in “theory only” … two 3" layers that were exactly the same ILD as a single 6" layer could be less durable over the course of a long lifetime because they will act more independently and abrade each other slightly but I don’t think that any difference would be significant or even measurable in “real life” terms and the other factors that affect durability (see post #4 here) such as the firmness of the layers would play a much bigger role. It certainly wouldn’t be a concern of mine.
Multiple layers would have more options for fine tuning though both before and after a purchase because for example a support core with a medium over firm layer would be a little firmer than a medium layer and a little softer than a firm layer (closer to the medium) and you would also have the option to rearrange the layers to firm over medium which would still be in between a medium and firm 6" layer but closer to the firm. You could also use the softer top layer in the middle to create a firmer feel compared to having the soft on top. The main advantage of having more layers in other words is that for those who need it it can provide more options to customize the layer combinations either before a purchase or by rearranging or exchanging layers after a purchase. While this is attractive to some people … it can also add some complexity that may not be necessary or may not justify any extra costs involved in having more layers for others.
Outside of a mattress with more layers having more options to customize the mattress, if a mattress is a good match for you then one isn’t inherently any better than the other.
Phoenix