I was trying to run two cars last year, the meega for cruising, holidays and clean jobs and the other one (a reet banger) for tools, dirt and city journeys.
I told various insurance companies that as I could only drive one car at a time please could they cover both. and not to worry about theft of the banger as it would be covered by
house insurance being worth nothing!
as others have said, they only would allow my NCD on one policy and the other vehicle would start from scratch with 0 NCD. making a £200 car cost at least £500 in insurance! (MoreThan have always been very nice to me on the phone and quite happily asked for nearly double other insurers quotes, so i stay away!!!)
Basically the only way to do it was to get a trade policy. Being a part time valeter is justification as motor trade! so my nan pays me to wash her car......hehe

However then i looked into it and found there are loads of extras missing on a trade policy (like courtesy car), the excess is higher and it was still about double just insuring the omega alone.
so i scrapped the banger, invested in a couple of cloths to keep the mig clean and am coming up to a £411 renewal for it...

the other way is to insure online and change car details online before you drive, not sure how often you can do it but i think in theory it's possibly to chop and change motors - paying the policy of the higher one of course!