Thanks for that. I just did an online quote, and it's come out substantially cheaper for my SLK and Fabia combined than it was for the SLK on its own at the last renewal. The overall quote for full-year cover on both cars represents an impressive saving of over £250, or 41%. However, I'll bear in mind some of the notes of caution expressed by others in this thread before deciding whether to switch.
I first looked into Admiral Multicar a couple of years ago, and the quote for my two cars at that time (the SLK and a C200T) came to a bit more than I could have got it for separately elsewhere, probably due to the fact that I had a few points on my licence back then. I ended up going with More Than, who offer to duplicate your NCB on a second car, which made a lot of sense at the time as I had full NCB on the SLK but was starting from scratch on the C200T. Not sure whether they'll do this for more than (no pun intended) two cars.
Someone suggested in this thread that Admiral also apply a single level of NCB across all cars on the policy, but it appears that this is not the case. They ask you to specify how many years you've earned on each car individually, and it states in their FAQs that if you make a claim (and your bonus isn't guaranteed or protected), then only the NCB for the car you're claiming against will be affected, so swings and roundabouts...
Also, from what I can see, the individual renewal dates for each car only apply until the next renewal, at which point you pay a pro-rata premium to bring the following renewal date into line with that for the first car you insured, so after the first year you end up paying a single premium on a single renewal date for all cars, which seems much more convenient (and cheaper to administer, of course).
However, Multicar only seems to be competitive for modern, day-to-day cars. I also have a couple of older cars on a classic policy, and adding them in to the quote worked out considerably more expensive that my current premium, despite having specified a low annual mileage for them.