Double Duplex & Roof Access question

I am looking at a brownstone which I’d like to convert to a 2 family, double duplex (e.g. st two duplex apartments stacked on top of each other). To maximize space, I prefer to use the existing staircase within the top duplex, but wonder if that creates a Fire Department access since roof access will only be through the top apartment and not a public staircase? Can the egress issue be solved by access to a sprinklered hallway and a staircase to the roof? Thanks, as always, for your words of wisdom.