Definitely have two AFMUs, one to repair the other.
Having more than one AFMU is good, but not to repair each other since the chances to completely wear out an AFMU are equal to zero and a 1% integrity AFMU is as good as a 100% one
No, it's better to have more than one AFMU, IF the internals space permits, for those scenarios when you take damage that is automatically distributed to ALL internal modules (forced supercruise drop offs, etc)
In that case is better to have 3 AFMU plus 1 Cargo rack instead of 1 AFMU and 3 Cargo racks - since cargo rack have no integrity so they dont take any damage, since in the first case the damage will get distributed to more modules and consequently each module will take a smaller share of the damage.
So, IMO, having more than one AFMU is good (if you have spare space), but not good enough to sacrifice other modules just for the sake of having 2 or more AFMUs