For 2 standalone maisonettes with an entry/exit via a single external metal staircase would a Fire Risk Assessment required?

and if so, practically speaking,  given the staircase itself is non-flammable what would an "assessment" entail?

If the single external metal staircase is used by both flats then that is a communal area therefore subject to the RR(FS)O but the FRA would have little to considerer other than the maintenance of the external, however the commercial premises below would have to consider the flats above when conducting their FRA.