I had an Admiral multicar policy until last week and switched to Direct Line. I had three cars on the multi-car policy 2 registered to me a 1 to my now ex girlfriend, it worked out for an Audi TT (roadster) Range Rover and a Fiesta about £930 a year the Audi and the Range Rover were fully comp.
I recently split with my girlfriend and made Admiral aware of the changes (Renewal time) and that I sold the Audi to my girlfriend so insurance only for the Range Rover and the Fiesta they quoted me just over £830
I said "so removing a group 19 car and a driver from the policy only saved £100" cs rep reply "oh but there was an accident last year with the Audi" my reply "yes my ex girlfriend was driving and the cost was less than £1200 and no damage to the Audi" they came back with "yes but you lose 3 years no claims bonus for that" even though I was not driving. I told them to shove it went to Direct Line got the Range Rover and Fiesta both insured fully comp with cover for travelling in connection with policy holders business and vandalism cover thrown in free, for less than Admiral.
Multi-car has it's good points and it's bad ones, you get two or three (depending on how many cars you have) sets of documents sent to you but it's also worth looking at other insures who will do a linked discount policy and can mirror ncd on both policies.