I found an all natural latex mattress, 5 or six inch core and 3inch comfort latex on top and bottom. More expensive, of course, but my question is; While you are using the first side, what is happening on the other? Are you really extending the life of the mattress?
When a layer is on the bottom it is evenly compressed rather than compressed more under the heavier areas of the body and less under the lighter parts of the body that have a larger surface area. The bottom layers will also compress less than the upper layers would if the same layers are on top of the mattress because the materials above them will absorb most of the compression forces and these forces also spread over a larger surface area as they travel through the mattress to the bottom layers. In effect the layer on the bottom is resting when it's on the bottom and any compression will be more evenly distributed over the entire surface. A two sided mattress will significantly increase the useful life of the top layers even with a material as durable as latex and it would be reasonable to expect an increase in lifespan of 60% - 70% (although not twice the durability).
Of course like everything else involved with mattresses there is also a tradeoff. With a two sided mattress there are soft layers on the bottom so these can't be too thick or they could risk alignment (3" is about as thick as you would want to go). Because the firmer support layers are in the middle instead of on the bottom there is less room to add progressively firmer layers on top (or you would have layers that were too soft on the bottom) and a mattress with the support layer on the bottom has more design flexibility to design in more complex layering on top and to fine tune a mattress for the more specific needs and preferences of different people.
If you have tested a two sided mattress and it works well in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) then any design benefits of a one sided mattress wouldn't be an issue. Some one sided mattresses also have a zip cover with individual layers and this can also be a durability benefit because you can just remove and replace an individual layer that has softened without replacing the entire mattress.
A two sided mattress also adds some extra costs to finish the mattress on both sides with the extra comfort layer and the quilting and cover on the bottom compared to a one sided mattresses that uses the same materials but it it can extend the useful life of a mattress significantly.
An update to my mattress story:
First I bought Tempurpedic Rhapsody Breeze as mentioned before: it sinks after around 5hrs and loses lower back support and it also heats up, so I...