Hi Frances,
Thanks for the kind comments … I appreciate it 
Your reasoning certainly makes sense to me. It also makes more sense as a starting point because of the weight differential between you. People with different weights will sink into a mattress differently and “come to rest” on different layers in a mattress. If you find that you need to add a softer top layer with your lighter weight the top layer can provide most of the additional pressure relief that you may need and the middle transition layer would “act” more as a support layer for you while for your husband the same two top layers would “act” more as comfort layers (he would sink into them more deeply and they would “feel” softer because of his higher weight) and his primary support layer would be the bottom layer in the mattress (which would have less effect on you because of your lighter weight).
A combination of soft and medium top layers would be less risky for him than if you were to initially choose a 2" soft layer on top of the firmer support core and then discovered that you still needed to add an additional 2" or so of soft latex on top of that (you may still “feel’ to much of the firmness of the very firm support core with only 2” of soft latex on top of it) and ended up with soft/soft/firm instead of soft/medium/firm. This is one of the ways that the same mattress can be suitable for couples that are in different weight ranges and have different needs and preferences (see the first part of post #2 here).
I’m looking forward to your comments and feedback once you receive your initial layers and have had the chance to sleep on them for a bit.
Phoenix