Why not just get a mirrored ncb policy...loads of companies do it? Fair play you lose the ncb on both if you claim unless you have protected ncb..but you would be able to have protected no claims if you have 6 years ncb anyway. I think you need at least four years to get it?
You can't use the same ncb on two separate policies as it's illegal. But as you can get protected no claims...why not get that, mirrored to two cars. I think you then get like four claims on a policy don't you before the protection runs out (sure someone can clarify this)?!
So giving you a legal, 6 years no claims on two cars that is protected if you claim on either car.