Does the Dunlap latex have more side support ? Can I put a 1/2" x 3" strip of plywood on the edge between the layers if necessary ?
Dreamfoam 12 in 1 Customizable vs Ultimate dreams Customizable latex ?
This would depend more on specifics of the mattress and the firmness of the support layers and the thickness and softness of the comfort layers than on the specific type of material in the mattress although latex is more “point elastic” than polyfoam so latex can sink in more deeply with more concentrated weight in a similar firmness level. You can see some comments about side support for latex mattresses in post #3 here.
Since Dreamfoam makes both mattresses a conversation with them would be your best source of guidance about the firmness of the side support of each mattress in different firmness levels or layer combinations relative to your weight.
You could certainly try it (or you could use a very firm thin layer of a more flexible material such as polyfoam which may have less risk of damaging the latex as well) but the negatives of how it affects the feel of the mattress could outweigh the benefits for most people so it’s not something that I would do unless your experience indicated it was absolutely necessary.