Hi Prof_Admiral_H_Anderson,
I don’t recommend “the best” of anything because what is best depends on the needs and preferences of each person. My goal is to help you with how to choose what is “best” for you regardless of how well it may work or perform for anyone else. “Best” is always relative to each person and their unique body type, sleeping positions, and preferences.
Yes … in theory you could put one topper over another but I think it would be much more complex in most cases to choose the combination that you preferred than just using one. I am a big believer in simplicity and it’s difficult enough to choose a single topper and try to estimate how it will interact with you and the mattress you are using it on much less introduce a second variable into your choices. At the very least I would choose one at a time.
Most toppers just lay on the mattress and then your mattress protector and sheets are enough to keep them in place.
Again there are no answers to these questions because each person may have their own preferences. The only answer is the one that feels and performs the best for you and has the best chance of providing the specific benefits you are looking for.
In terms of quality and value (which is all I can really speak to) all the toppers you are looking at would make good choices but only your own experience on the mattress/topper combination you choose can tell for certain how well it will work for you in combination with your “firm” mattress (each person has their own definition of what a firm mattress is)
The best way to choose a topper is to test the same or a very similar mattress/topper combination locally so you can use your own experience to know how well it works for you.
If that isn’t possible or practical … then the guidelines and links in post #2 here in combination with your conversations with the manufacturer or retailer of the toppers you are considering so you can take advantage of their knowledge and experience about the products they sell is the best way to decide on which topper you believe would be the most suitable for your needs and preferences and which one is the best “fit” for your “personal value equation”.
In the end … each person is unique and nobody else can tell for certain what you will feel on any mattress or mattress / topper combination so personal testing and/or using the guidelines and information available to you and talking with people on the phone is the best way of increasing your odds of making your best choice.
Post #4 here has a list of component suppliers and if you look under covers and talk with them then if you end up needing it you would likely find one that will make a 5" cover that could be used to cover two toppers … but I would personally only work on one variable or topper at a time and when you have slept on that topper/mattress combination for long enough (I would suggest a couple of weeks at least) then use your personal experience on that combination to decide if you need a second one.
Phoenix