In the cruise one ADIRU is connected to one AP to fly the plane. If this ADIRU gives faulty information the AP will follow it until disconnected.
A year ago I was trying to troubleshoot an ADIRU problem on an A319. The crew had seen the problem and selected the AHSI to nbr 3, but had not connected the AP to nbr 3 ADIRU. I can't remember exactly what it was, but it took a lot of detective work to confirm what actually happened. The crew thought that they had selected AP to ADIRU 3, but had actually only selected the display, not the AP, so when the fault reoccured they assumed that two ADIRU were at fault.
In the end there was no ADIRU fault at all, the fault was in an ADM (air data module)