I can't speak for the 57 but when I flew the 34 I understood the single pilot thing was due to a single instrument source i.e. both pilots are looking at the same data. What about the 57/18/45? The 44 has two sources (one for each side) so there is a level of redundancy the 34 doesn't have. So not really the level of copilot mattered as much as the data being presented to that 2nd pilot.
That makes sense, but I'm waiting for a HT guy to chime in. My recollection of SOPs and things Wiener oriented are fading fast.