Hi Braphie,
Weight concentration is more important than overall weight and you are well inside an average weight range. When one or both people are heavier they compress the foam much more on their side and this mechanical compression would have an effect on durability. The upper layers always compress more than the deeper layers so the effect of heavier weights on durability and which layers are most affected depends on how deeply into the mattress someone sinks.
You can see a comparison between a latex hybrid and an all latex mattress in post #2 here. The durability difference with two layers would be less (the lowest layer is the one that compresses the least) but there would still be a performance difference between them because latex is more adaptable to different weights and sleeping positions but the difference would be less with two layers of latex than with only one.
You can also see some thoughts about thicker layers or mattresses in post #14 here and more latex vs less would have similar benefits.
So it really is a tradeoff that depends on your own budget and on the importance of performance benefits that for most people (unless they were quite light) would be less but still noticeable.
Phoenix