If I had 2 cars on 2 seperate policies and had an accident in the one with no NCB, would it affect my other policy with some NCB?

If you had an accident and you were at fault, this will be taken into consideration when calculating your quote because you will have to disclose the accident if it happened within the last 5 years. So, although your 5 years NCB on the 1st policy may not be reduced, your premiums would still be affected because your circumstances have now been changed.

Your final quote is calculated after applying your NCD to your premiums. So if your premiums were supposed to be £300 and you have 5 years NCB (worth 60%) to apply on that, you would pay £120. But if the premiums were to increase because of a fault accident, then the overall quote would also increase even if your NCB stayed the same.