You can only use the NCB on one car at a time. So the above Aviva quote would be invalid.

There are however insurers that will apply the same level NCB to your new policy after seeing proof of your existing NCB on another car, or as you already know your existing insurer will mirror the bonus onto your new car.

I have used firstpoint insurance specialists who found an underwriter that would duplicate my NCB onto a second car. However should I move that second cars insurance away from them, I would only receive the NCB for how many years its been insured with them. This will be the same with your green light mirroring.

i.e. you have a car insured with them with 8 years NCB.
They mirror this bonus onto car 2.

If a year later you move car 2 onto another policy with another insurer, you would only get 1 years NCB from greenlight for that individual car. Car 1 would have 8 years NCB unaffected.

Hope this helps.