I think you need one of these multi-car policies; not heard of anyone applying NCB to two cars at once before.. Might be worth checking if you can get a driver policy though; I think a-plan are quite good at that, some of my mates have insurance for a road car and a track car through them which Im pretty sure takes into account the NCB on both, although there might be some random clause like at any time, one of the cars must be garaged or something..