Hi brass,
I would agree with dn’s comments and there is also more information about the pros and cons of two sided mattresses in post #2 and #3 here and the posts it links to.
I would also read post #15 here and the posts it links to before considering any DIY mattress project to make sure that you have realistic expectations of success and cost.
Designing and building a one sided DIY mattress is generally difficult enough (unless you are using a proven design that you know is suitable for you and you have access to the same layers and components as your reference mattress) but building a two sided mattress would be a more specialized mattress design and construction with much more unpredictable results and I don’t think it’s a project I would take on without a great deal of experience and knowledge about mattress theory and construction and some extensive testing on different mattress designs as well. I think that a two sided design would also benefit from using a tape edge finish (rather than a zip cover) although this isn’t something that could be done as part of a DIY project because it takes specialized equipment and I would also consider gluing the layers as well.
Phoenix