I don't understand this logic.
The insurer is insuring the person more than the item. So if a person owns 2 cars, he's only spending marginally more time on the road than with 1 car, so he's not really an additional risk.
But if there are 2 drivers, having 2 cars means more time on the road, so the risk is increased.
Perhaps having both cars on one policy will be cheaper because the insurer is already taking on part of the risk, and now is assuming only some additional risk, instead of a new policy where the new insurer is taking on new risk, (but the old insurer would now be removing the 2nd driver from their risk?)
Deal Guy, ask your broker or insurer. Doesn't cost to ask.
Please get back with us so we'll know - at least in your case.