Good point. Why do we even bother with two-lane roads and a double yellow line? Such a waste of space. Very contrived to presume there is always a car coming the other way
You're comparing a situation that happens all the time (opposing traffic) with one that happens extremely rarely (blocked stairwell in a fire). If anything, you're strengthening my point.
We should design for situations to a level that is appopriate given their frequency and severity. Show me evidence that MANDATING the extra stairwell justifies the huge increase in national housing cost, and I'll concede.
Sorry, but the only thing that will change my mind is a significantly casualty different between single stairwell and dual-stairwell buildings accounting for building age, construction type, property value, and occupant demographics.
I just dont think it would really help that often. It is not the stairwells that are burning, it is some appartment on a floor below. Your problem is going to be smoke and visibility, not some couch blocking the stair. If those stairs are connected, you will most likely have smoke everywhere and will have no clue if one stair is safer than the other.
We can avoid this dangerous scenario with a three-stairwell minimum requirement.