Hi tavak,
You may have read this already but just in case you haven’t there are some topper guidelines in post #8 here that may be helpful.
Your own personal testing and experience is really the only reliable way to know which topper / mattress combination will work best for you because each person and mattress/topper combination is unique. Besides your own body type, sleeping positions, and preferences, … thickness and softness will both have an effect on how a topper feels on any particular mattress. Thinner toppers will “allow” more of the feel of the mattress underneath to come through while with thicker toppers you will feel more of the characteristics of the topper itself and they will generally feel softer. Thicker / softer toppers can increase the risk of having an alignment issue because they can allow the heavier hips/pelvis to sink down further before reaching the firmer support layers so “just enough” to relieve pressure in all your sleeping positions is the least risky approach for alignment. The key is always to balance your need for more pressure relief (with a combination of thickness and softness) with your need for good alignment.
Both 19 ILD and 24 ILD are in the soft range and would be suitable for many people in your weight range. Some will prefer one and some the other. Of course 24 ILD will be a little firmer but there is not a huge difference between them. There isn’t a formula that you can use to accurately predict any specific person’s experience.
As you mentioned you can also order two twin XL toppers and use a different ILD on each side but you may need a mattress pad or something a little thicker than just a thin topper cover to mask the feel of the split in between them. If the split combination works well then you can also glue them together using foam glue that you can get from a fabric ship to prevent them from separating where they are split.
If you let me know your city or zip I may know of some stores in your area that sell latex mattresses that you can test that may give you some reference points than can help you decide on which choice you may prefer. Without some reference points there is really no way to know which one you will prefer and you will need to decide based on your “best guess” and “best judgement”.
When you are undecided and only personal experience can really know which one will work best then the exchange policy for the topper may also be an important part of the “value” of your purchase.
Phoenix