These ('scissor stairs') are an important part of the discussion here. They are allowed (or counted as two separate means of exit) in New York, Australia, Vancouver, etc. but not permitted (or counted as one means of exit) in most of the rest of the USA, the UK, etc.

For a high rise building they don't take up much more space than a single stair and can play an important role in the structure of the building also.

They are called scissor stairs and it's very common here in newer buildings in NYC as well. Though I believe it's only permitted in buildings 12 stories are less? It always seemed very space efficient -- separated by concrete fireproof walls and taking up the same vertical space.

Sounds like a death trap.