Compass redundancy sounds great, but it doesn't actually really work like that. Today, ArduPilot does not have a process to detect a compass that going bad, reject it, and use another one instead. That would be great, but we're not there yet. That among other things would be great improvements on the compass handling
All it can do today is look at them both and monitor for consistency. If they become inconsistent due failure or calibration problems, it will generate an EKF error and stop using them altogether. It cannot pick one or the other. It can only tell you "hey, something is wrong with my compasses".
As such, the more compasses you introduce, the more likely it is one will fail or get confused, and the more likely it is to cause a problem rather than solving one.