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.
Twins such as the 757, 767, 777, etc can hold the cabin just fine at max cruise altitude for one reason.... ETOPS. That way the loss of one pack won't force a descent while in the middle of the ocean... you can keep on flying at your planned cruise altitude. But if dispatched with a pack inoperative, you'll have to limit flight to a lower cruise altitude, although it is still much higher on those airplanes (FL350 for a 757).