Apparently by code they could get away with a single stairwell until quite recently. That said, would an extra staircase have helped, given the the conditions would have probably filled both with smoke rather quickly?
Really, the only thing that might help would be to have interior space dedicated to chutes or something to the ground floor for escape.
At least one of the taller buildings I worked in several years ago has a completely separate ventilation system for the main stairwell, all entry doors are steel exterior doors opening into the elevator lobby at each floor, and propping them open was (at least at the time) one of the few ways to really get on management's bad side in a hurry. With nothing inside the (concrete and steel) stairwell to burn, and as well isolated from the rest of the building's air as it could be without some really specialized doors/airlocks, it should stay usable in a fire up to the point where one or more of the lower doors burn through.
Interestingly, that one was also triangular, with enough space down the middle to hoist equipment up that way rather than using the cargo elevator. An electric hoist with its own backup power (main purpose was to hoist fuel drums to the rooftop generators) was on the roof over a large hatch, and according to the overnight guard (after being an overnight guard for a few years myself, I now understand why the guy was so talkative) at least a couple of the maintenance guys on each shift were trained and regularly drilled in using the hoist and a litter to load and lower an incapacitated person safely from any floor to the basement tunnel access where they had a straight shot to a loading dock. (Sounded fun, since one of the guys would have to ride down in a harness alongside the litter to make sure it didn't hang up on the stair rails, and control the landing. Raising inanimate stuff was apparently easier, as it wouldn't move on its own to impart swing, and a bump would mean at most a little touch up paint on the stair rail.) The other stairwell was a normal zigzag, and opened to secured parts of most floors, so it was "outbound only" unless you had a master key.