Not all twins can maintain cabin pressure at cruise altitude with one pack. The MD80 definitely can't. If you lose a pack in an MD80, you are required to descend to FL250 in order to maintain the cabin. This happened to me once while at FL330, and the cabin altitude began climbing around 300-400 fpm immediately. We descended to FL250, but that was not even enough. The cabin still was climbing. We then descended to FL230, and found that we could hold the cabin fine at that altitude.
Was that a MD81?? Because i can guarantee that a well sealed MD80 keep the cabin ok with just 1 pack. It happened to me and I didnt even notice, beacuse mad dog doesnt warn you the pack shut off