Gizmo:Flying the Cessna Citation both as a crew and single pilot I feel qualified to comment on this subject.
When a crew is working well together in a standardized an competent manner I feel it is much safer to have a two pilot crew.
That said if one is saddled with a non flying pilot who is not competent and does not perform their tasks properly it may be more trouble to watch over them than to just do all the work yourself. That way you at least know what has been done and that it was done to your standards.
When flying for an airline the training and standards usually insure that the level of crew coordination makes a two pilot crew safer. That is not always the case at some corporate operations where the level of training and standardization may not be nearly as great.
It is interesting to note that while attending Flight Safety International the instructor told me that single pilot Citations have enjoyed a better safety record than those flown by two pilot crews.
Sincerely, Stearperson