simple answer ! get married then you swap ncd between husband and wife.
the car that you drive the most is not a problem just insure normally.
the other car that your partner drives the most you need to find a company that will let accept you as the policy holder and use your second set of ncd even though your partner is main driver. alot of big companies do this like direct line and nunion but ask at begining of call.
once you have been with this company one year the renewal will show partner as main driver... other companies who then base ncd on main driver will then accept this renewal as proof of ncd. will take two years and you might not get cheapest this year but certainly cheaper than getting married or starting your partner on 0years ncd