Hi Barra,
When you are testing a mattress locally I wouldn’t pay any attention to the ILD of the layers and I would use the results of your testing to decide on whether a mattress is a good match for you in terms of PPP. While it’s always important to know the type and quality of the materials so you can identify any potential weak links in a mattress and make more meaningful comparisons , the “comfort specs” such as ILD are really not important.
It’s not “necessary” no but it can extend the useable life of any mattress for those who flip it regularly even with materials that are as durable as latex. There is more about the pros and cons of a two sided mattress in posts #2 and #3 here and the other posts it links to.
The deepest layers of the mattress don’t compress as much as the upper layers because most of the pressure is absorbed and “spread out” by the layers above them before it reaches the bottom layer but if the comfort layer on the bottom is too thick and soft then this could have an effect yes. Making sure that the comfort layers aren’t too thick and soft is one of the limitations of a two sided design. Once again your own personal testing will be the best way to know whether a mattress is a good match for you in terms of PPP whether it is one or two sided.
This is one of the advantages of a component mattress where you can replace the individual layers if one layer softens before the rest or if your needs and preferences change. It would also be an advantage of dealing with a manufacturer that can open up a mattress and then replace an individual layer and then close it up again if that becomes necessary. While it would be more costly because you would still need to buy the replacement layer, it would still be less costly than replacing the entire mattress. All of these can be good options depending on the parts of your personal value equation that are most important to you.
Regardless of the individual options … all the mattresses you are looking at use good quality materials and have no weak links in their design.
I’m not sure if you mean a mattress where the internal layers are split with different layers on each side or if you mean two twin XL mattresses that are put together and used as a king size.
Post #2 here has more about the pros and cons of split layering and post #2 here and the post it links to has more about the pros and cons of loose layers and the risk of shifting in a component mattress vs a mattress where the layers are glued.
If you mean a split mattress where you are using two separate mattresses together then there would be a more noticeable “gap” in between the two mattresses.
Have you asked the two manufacturers you have been working with?
There is also a list of some of the members here that sell online in post #21 here and some of these also make a mattress that would be similar to the design you described.
Phoenix