This is actually an issue that bugs me as well. Just remember that if the policy does not run for 12 months then you will not accrue any NCD.
I have a multi-bike policy, 2 bikes, one for the road, the 2nd a trail bike (which has to be road legal). I never ride the road bike in the colder months, it's a summer toy basically, and yet the road bike, being worth considerably more than the trail bike, is what makes up the majority of my insurance premium. I enquired with my insurers whether there was anyway to take this lack of use into consideration and reduce the cost accordingly, i.e, reduce road bike cover to fire and theft only for 5 months of the year, guess what??, no way!!. In the same vein I asked whether reducing the annual mileage on the trail bike from 6k to 2k would reduce costs, again no way, both bikes have to have the same limitation. So all in all it's just another way for the insurance companies to screw is over. Yes I could insure them seperately, but then full NCD only counts for one bike, the 2nd bike has zilch NCD so no financial benefit.