Hi scbrooks,
Outside of some basic “needs” … pillows are much more about preferences and how they “feel” than a mattress where the “needs” are more important and a bigger part of your choice.
As long as a pillow (or a combination) keeps your head and neck in good alignment in all your sleeping positions (or are “scrunchable” if you sleep in different positions that need different thickness and support choices) then the choices between different types or combinations are strictly about preferences because the head and face is much more sensitive to differences in how a pillow feels than other parts of the body. There is more about pillows and some good links to more information and feedback about different choices and materials in the pillow thread here.
So the bottom line is that I don’t see any reason not to put a firmer probably flatter pillow under another one if it works for you in terms of loft, support, and alignment and the overall “feel” of the combination (and you would feel more of the top pillow or course than the bottom one so the feel or quality of the bottom one may not be as important). You could also put them in the same pillowcase to keep them together although then you could only use one side if you are a “pillow flipper” and you liked the “feel” on the latex more than the other side of the combination.
Phoenix