This is an example of situation where it is essential to have a conversation with new insurers.

I now wish to insure the two cars separately with other insurers. Am I OK to declare 6 years NCD for the second car even though I have only owned it for 8 months?

You will probably be asked to get confirmation of NCD from your present insurer, so it will be up to them what they confirm.

I had exactly the same issue when I bought my first car, a 90s Nissan Micra, from a main dealer back in 1998. It came with some nice 'freebies', including free RAC breakdown cover for a year, MOTs for life (but I had to use Nissan dealers and show my freebie card they gave me) and an introductory £350 insurance policy with the RAC, which was good even back then for a new driver (you did have to be over 20 though).

The problem for me was, like you, the policy had a 'free' 5 years no claims bonus and bonus protection thrown in, but that only mattered on year 2 if I stayed with them. All the other insurers said I would be treated as someone with only 1 year's no claims. This may have changed in 20 years, but I doubt it. It meant that all other premiums were WAY more expensive, meaning the RAC could afford to jack up the price for another three years becaue everyone else was still too expensive.

It was essentially the same wheeze that some equipment manufacturers play in the Construction Indutsry, epsecially controls systems firms who hook unsuspecting clients in with cheap equipment and installation costs, and then shaft them with huge maintenance and parts costs after the 1 year warranty runs out, keeping them shackled at a rip-off price for maybe 10 years or more.

In this case, it may depend on how old and experienced the OP is as a driver and how many claims they've made generally in the last 5 years.

UPDATE: Spoke to two potential new insurers today (Churchill and Chris Knott) and both are happy to accept the 6 years NCD shown on the renewal letter from the current insurer (Aviva) even though I have only owned the car for 7 months or so.

One of them (Churchill) said they weren't interested in the origin or history of the 6 years NCD provided I send them the 'Confirmation of NCD' statement from Aviva that was included in the renewal letter.

Thanks to everyone for the forum replies.