I'm pretty sure that the data source for ISIS is selectable, just as the standby instruments were in the steam gauge days. You seem to be concerned that there is a computer involved - why? The computer is not generating the data for the EFIS, it is merely translating it for display.
In the case of my experience, the ADC on my side was failed, and the ISIS was taking it's information from the Captain's ADC, which was why I could read it.