I'm pretty sure that the 2 pilot operation requirement is for aircraft who's MTOW is 5700kg or more, and the aircraft is on an AOC and operating on public transport op's.
Also, both pilots have to be rated on the aircraft type, (in the early days this wasn't a requirement, and there were a number of operators of E110s etc who utilised one fully qualified pilot and a pilots assistant, who could be no more than a basic PPL).
Many operators of privately/corparate owned aircraft utilse two fully qualified crew at all times, regardless of weight, due to the complexity of systems etc.