Where are you getting that "one will cancel out the benefit of the other"?
From what I gathered, the study looked at getting either AV or
MV replaced, not both together.
The study also found that unlike what’s recommended in the national guidelines, which say patients 50-70 years old undergoing aortic or mitral valve replacement should be given a choice of either a mechanical or biological valve, the best choice in fact can hinge on whether the aortic or mitral valve is being replaced.
As per this study, it says for AVR, "the benefit of implanting mechanical valves ceased after the age of 55."
So at your age of 65, this is an indication for biological AVR
But, for MVR, it says "a mechanical valve is actually beneficial until the age of 70". So
at your age of 65, this is an indication for mechanical MVR
(as you are 5 years away from 70).
But in your case, I don't think you can interpret these indications in isolation as you say you need both valves replaced. i.e. you wouldn't get a biologic AVR and a mechanical MVR in the same intervention, would you?
Multi/mixed valve disease is more complex and presents a different set of challenges that need to be discussed. Take a look at section 10: Mixed Valve Disease
in the below link:
2020 ACC/AHA Guideline for the Management of Patients With Valvular Heart Disease
I think you mentioned you are going to CC so you will be in good hands and will be guided to make the best decision in your case.
Maybe one or both valves can even be repaired?