Everyone's experience is different i guess. One experienced ferry pilot I contacted said it would not be a problem carrying a co-pilot/safety pilot/relief pilot on a single pilot turbine twin, it depends on the insurance company. Whether you want a spare pilot is more of a personal preference, as there is enough space on most turbine twins and singles for you to pack a life vest/raft or wear a suit and carry another pilot right seat.
But I'll go direct to a few insurance company's and find out. That will cut out any speculation on the topic.
thanks anyway