The thing that has to be remebered is this; if two people insure a car and it is written off or stolen or whatever only the legal owner stands to lose out. Therefore a vehicle cannot be insured twice as if it is stolen two amounts of money would be paid out and one person would be in a better position.
If a vehicle could be insured twice then I would insure my next door neighbours XC90 which is worth £30k and pray it was stolen!