Before dispatch with one PRSOV inop, that valve is secured closed by the maintenance engineers. On the other hand, if it fails in flight, there is no sure way of knowing that it will stay closed throughout the flight since it is inop and might disagree with the switch position. If it opens midflight, then both the bleeds will be supplying air to the left pack which is not allowed and might damage the system. So, unless the inop PRSOV is secured closed, we keep the isolation valve closed.
I hope this helps.