I agree with tommoutrie: privately, if the a/c is licensed for it and you have the endorsement on you licence, you're legal.
The insurer insures the a/c to be operated legally. If there are any experience, qualification or operating exceptions then the operator should have been made aware of them when the insurance was written. Anyway, most such exceptions are instigated by the operator, as a way to lower premiums.
But I'm yet to get a convincing reason why you wouldn't just want to fly two crew?!