You might consider 2" of latex and 2" of poly or memory foam. Firmness would be determined by your height, weight, and sleep position. More on reddit.com

Density really does matter in a mattress topper. I have many, many different toppers of different densities and my favorite, by far, is the 4lb topper from Foam Factory. They can send you sample cubes ofwhatever you like and at least you can feel the difference in “squishiness” between them. The 4lb has some heft to it and feels much more supportive than the 2.5lb and something like Serene foam. I also had a Viscosoft topper, and that, at 3.5lb density was way, way too firm for me, but I weigh 132 and am a side sleeper. It was an excellent quality topper, and I can see it working for a lot of people. More on reddit.com

You’ll need a Twin XL for a split king. I have this topper from Foam Factory, 3 inches of 4lb, and it is absolute heaven for me as a lighter weight side sleeper, with a bad hip. In fact, my hip pain is 90% better after buying this topper! I have only had it for 4 months, so I can’t speak to longevity, but I am very pleased to have it. No off-gassing whatsoever, which was a huge bonus. More on reddit.com

Its going to feel very firm and you will feel the firmness of the floor through the latex. The pressure on your shoulder and hip would be unbearable for most side sleepers. Adding more soft comfort layer isnt the solution either, the support(Your floor) is just too firm. What will likely happen you will either feel the hard floor underneath, until you have added enough soft material that instead of your shoulders and hips hurting your back will hurt from bad alignment. There is a really inexpensive solution though if youre trying to save money. Just get 5" of the HD36 High quality foam for $135 in queen and you have yourself a mattress. They also sell Lux HQ foam which is extremely firm at 50ILD, he might like that better, but I would think the HD36 HQ has a better chance of working for both of you than the Lux. Edit: Another comment suggests a want to stay organic, having a memory foam topper suggests otherwise but I figure I can expand on my advice anyways. Probably the cheapest option keeping it somewhat natural would be some Caliber edge coils for $230. Then you just set the topper on that and you have a latex hybrid. All of this is DIY territory though so it should also be noted that most of the time when you go down this route you will not get it right the first time. You usually have to make adjustments, eventually you want to get a mattress encasement to hold it all together, it can be quite a thing. This guide here is a good starting point if you go down that route. Also if you end up going with the Roma, just keep in mind that soft side is more like a med-firm side, and it isnt a natural mattress at all either. It has supersoft foam quilted into the panel. More on reddit.com