It aint that simple.
Both the King Air and the DHC6 Twotter (for example) are certificated as single pilot but in most areas the local CAA requires 2 pilots when in commercial operation. Some allow single pilot commercial operation VFR only, others have more complicated rules.
So - it depends and you have to know the local rules.
One interesting side effect is that a pilot can only log P2 when operating in conditions where the second pilot is required by the rules. Its a mess.