Hi mainertodd,
Like most questions that have to do with mattress design and theory … the only meaningful answer would be “it depends”. In this case it would depend on whether you are talking about firming up primary support (which generally involves the deeper layers), secondary support (which generally involves the upper layers), or the “feel” of the mattress (which generally involves the top layers). There is more about this in my previous reply about primary and secondary support, pressure relief, and “feel” and how they interact together.
It’s not unusual at all for a couple to have different needs and preferences and there is more information in the first part of post #2 here about some of the different ways that can be used to accommodate a couple that have very different body types or different needs and preferences in a mattress.
There is also no problem with having different firmnesses for each side of a mattress and there is also more information about the pros and cons of split firmness mattresses in post #2 here that may be helpful.
Different people can have very different perceptions of firmness and softness compared to others and a mattress that feels firm for one person can feel like “medium” for someone else or even “soft” for someone else (or vice versa) depending on their body type, sleeping style, physiology, their frame of reference based on what they are used to, and their individual sensitivity and perceptions. There are also different types of firmness and softness that different people may be sensitive to that can affect how they “rate” a mattress as well (see post #15 here) so different people can also have very different opinions on how two mattresses compare in terms of firmness and some people may rate one mattress as being firmer than another and someone else may rate them the other way around. This is all relative and very subjective and is as much an art as a science.
A person that is heavier may sink into the mattress more deeply and feel more of the properties of the layers that are deeper in the mattress while others that are lighter may only feel more of the properties of the layers that are closer to the top of the same mattress.
All the layers and components in a mattress (including the cover and quilting) will affect the feel and performance of every other layer in a mattress above and below it and the mattress “as a whole” to different degrees so what you feel on a mattress is the combined effect of all the layers and the “mattress as a whole” more than the effects of just a single layer but in very general terms the properties and firmness of materials and components that are closer to the top surface of a mattress will tend to have a bigger effect on the overall “feel” and firmness of a mattress than materials that are deeper in the mattress, thicker layers will contribute more of their feel and firmness to the mattress than thinner layers, and a thinner layer would “allow” more of the feel and properties of the layer underneath it to “come through” than a thicker layer.
Both a “medium” layer over a “firm” layer (however you define medium and firm) and a firm layer over a medium layer would “feel” somewhere in between medium and firm but the firm layer over the medium layer would be more towards the firmer end of the scale and the medium over the firm would be more towards the medium end of the scale.
Phoenix