You can only apply one set of NCD to a car insurance policy. So if you had used your no claims bonus on the 1st policy, you wouldn't have any left for the 2nd policy. Now the problem is that when you lose your NCB, it gets wiped off everywhere whether it's a policy of your own or not.
In your case, if you had a fault accident, you will lose your no claims bonus and that depends on how much you had to start with and that loss will need to be reflected in both the policy where you have a second driver as well as the one where you're on your own.
No matter what, you still need to inform both insurer to tell them you have a claim now as this is a change of circumstances and car insurance companies want to know about this, otherwise they can invalidate your policy.