Phone Coop again. Explain the situation. You cannot use the NCB twice. Coop think that you are trying to use the NCB you have with them, to get a cheaper policy elsewhere for the second car.
Many people think that the NCB is something you earn and can then be used on as many policies as you like. This is not the case. If you own one car and have earned 5 years NCB, you can only use it on one policy. If you buy a second car and Insure it separately, you won't have any NCB on the second car policy, unless the second car policy Insurers are allowing you a special deal.