Hi Annmarie,
As dn mentioned there really isn’t an effective way to make soft comfort layers any firmer outside of removing them and replacing them with either thinner layers (that are closer to the firmer support layers of the mattress) or with firmer foam. It’s much easier to make a mattress that is too firm softer because this can be done by adding a topper. There is more about this in post #4 here.
A one sided mattress isn’t designed to sleep on the bottom side of the mattress which would generally be much firmer and designed to to support the mattress on a foundation not to sleep on directly but if you add a suitable topper then it may be a better option than replacing the entire mattress. There is more about choosing a topper in post #8 here.
In general stomach sleepers need a much thinner and firmer comfort layer which is probably why your husband is “OK” with sleeping on the bottom but side sleepers need a thicker softer comfort layer to relieve pressure points on the mattress and for good spinal alignment that can “fill in the gaps” in your more “curvy” side sleeping body profile.
You would need to either compromise on a topper in terms of thickness and softness or you could also use two different twin XL toppers of the same thickness on each side of the mattress (two twin XL’s are the same size as a king size) and use a different firmness level on each side to better accommodate your different needs and preferences. Much firmer latex may be a good choice for him (especially in a thickness that would be more suitable for you) because of his stomach sleeping while softer latex may be a better option for you to accommodate your side sleeping and pressure points. It’s generally a good idea to use the same type of material on each side so there isn’t as big a difference or “transition” in the “feel” and softness level from one side to the other in the middle of your mattress.
It’s not a perfect solution but it may be better than replacing the mattress.
Phoenix