However, it is likely that a 2-pack airplane is allowed to dispatch with 1 operable pack. In that case, a second failure is not unforeseeable...
Very true, but in that case we are limited to FL250 max (at least in the 737). As others have said, the outflow valves will close as the
cabin altitude starts to rise. In the event of a climbing cabin with both pack lights illuminated and inextinguishable the QRH calls for a descent to 10 000' or the highest safe level. At 10k the outflow valves can be safely opened and the depresured AC can be continued or diverted as decided by the captain - although at 10k, regardless of company SOPs, a fuel diversion would be the likely probability.
RIX