They use scissor stairs. That's two stairways in one stairwell, with the stairways winding around each other. They count as two exit stairways if (1) the stairways are enclosed in and separated from each other by masonry or masonry equivalent construction having at least a 2 hour fire resistance rating, and (2) the exit doors are 15 ft or more apart.

While they might work as far as providing residents two routes out in an emergency, they do present challenges to firefighters [1]:

> Jack J. Murphy, a former firefighter, fire marshal, and current chairman of the New York City High-Rise Fire Safety Directors Association, has inspected some of these buildings and says they present unique challenges for firefighters. For one, the buildings’ small footprints mean very narrow scissor stairwells that can have five or more turns between floors. “What would that do to my hose stretch?” Murphy said. The compact size also means tight quarters for operation and medical staging in an emergency.

[1] https://www.nfpa.org/News-and-Research/Publications-and-medi...

Interesting, can't imagine going down over 1000ft of stairs without sliding on my ass, much less going up with a fire hose in heavy gear.

A scissor stair can fit 2 stairways very compactly, looks like this is used on 432 park ave, for instance.

https://images.app.goo.gl/kz1826Y8xydc9G9L8

I'm going to guess that codes don't require two stairwells if there is only one apartment per floor.