Hi shaungregory,
This could certainly be a good plan so you could use your toppers as part of your sleeping system. There is more about the difference between using a topper vs having the same layer inside the mattress cover in posts #3 and #4 here and in this topic.
I would agree that the new Brooklyn Bedding mattress uses high quality materials that would be suitable for your weight range in terms of durability.
If you were to purchase two twin XL mattresses that are the same mattress then they would of course fit your matching size requirements. There is more about the pros and cons of split king mattresses vs a single king size mattress in post #8 here and in this topic.
The Brooklyn Bedding mattress may work well for you by itself (which is what it’s designed to do) or you may find you prefer it with your topper on top of it. Every layer and component of a sleeping system will have some effect on every other layer and component and on the mattress as a whole and the quilted cover would add some additional thickness and softness in the top 7" of your sleeping system vs the same combination that had a thinner cover but you wouldn’t be able to “isolate” the effect of the cover unless you were to make a side by side comparison with the same mattress that had a different cover with the same topper on top of it because you “feel” all the layers of a mattress as a whole not individually. In the same way the only way to know whether the mattress by itself or the mattress with the topper would be the best “match” for you or which one you may prefer would be based on your actual sleeping experience.
While you would be ordering the firm anyway and then deciding whether adding the topper would be better than the mattress by itself, in your wife’s case if she is adding a soft topper then it may be worthwhile considering the medium and then using the soft topper on top of it so that there aren’t two “soft” layers in her sleeping system which could be risky in terms of maintaining good alignment.
If you do decide to go in this direction It may be worthwhile just ordering your side first so you can decide whether the mattress would be a better match for you either with or without the topper and then you would be in a better position to decide whether to order a soft mattress for your wife (if you prefer your mattress without the topper) or a medium for your wife that she can use with the soft topper (if you prefer your mattress with your topper) so that you can match the height of hers with the option that works best for you.
Outside of the benefits of having component layers that you can rearrange or exchange and the ability to replace a single layer if one layer softens or breaks down before the others (usually the top layer) or if your needs and preferences change over time … there is more about a latex/polyfoam hybrid vs an all latex mattress in post #2 here. The other advantage of a component mattress is that you can “leave out” one of the layers that you would otherwise purchase because you already would have one of the layers while with a “finished” mattress you wouldn’t be able to “leave out” one of the layers and if you find that you prefer the mattress by itself you would have a “left over” topper.
Your choice between the latex hybrid option either with or without a topper and an all latex mattress is really a budget and a preference choice since either one could work well for you “in theory” so I really don’t know which one would be better for you and it would really depend on which direction you feel best about based on all the parts of your personal value equation that are most important to you.
Phoenix