Flying single pilot requires a very specific skill set. Some might say over time it also can develop a certain style or attitude!
Flying true twin pilot requires additional skills, some of which can be difficult to acquire and not necessarily intuitive for some. That is why there is a requirement for a Multi Crew Course for those who's futures lie in that direction.
Shy's link nicely covers the concept of a 'Pilot's Assistant' who whilst somehow 'qualified' is most definitely
NOT a second pilot in the sense in which some may like to interpret it. I believe they can't log P1 or P2 hours?
Which brings me to chopjock:
a non type rated safety pilot along for the ride
WTF is a "non type rated" "
safety pilot" and what's safe about it?
Does some such thing exist within the legislation? Or is this simply a way for a PPL (director of the supplier) to justify his 'ride along' presence to a cameraman customer who knows no better?
It all comes down to precise definitions, but a PPL might not even qualify to be a 'Pilot's Assistant' on a 'Hire and Reward' flight? I don't know.
This is where lines get blurred.