I had the fortune of walking by a specimen of that rather nice piece of automotive engineering and of course stopped to admire it. It caught my eye that the rear wheel had two brake calipers - one of them being an enormous beast, as could be expected, and another one looking more like something for a small motorcycle.
My Google-fu is weak - what is the 2nd caliper for?
Parking brake.
Why not just use the regular one?
Because a caliper that has to operate both hydraulically (for the regular/service brakes) and mechanically (for the parking brake) is a complicated affair that is problem-prone.
Just a guess: Most cars have single cylinder calipers at the rear but the R8 has 4-pot calipers. That would require a tremendous force to be applied for the parking brake to be effective. The tiny caliper would have enough bite with minimal force.
Also the parking brake on the R8 is electric, operated with a button, like the VW Passat.
BTW, most old Citroens like the DS had also separate calipers for the parking brake. In the case of the DS, the parking brake (activated with a pedal like Mercedes cars) acts on the front wheels and is capable of stopping the car in an emergency, and for good reason. The DS had a single hydraulic system that controlled the steering, the suspension and the braking. If one of the gazillion tubes started leaking and you lost pressure the only way to stop the car would be with the parking brake.
Parking brake makes a lot of sense - probably should have figured that out on my own. (For some reason, descriptions of this sort of car tends to be lacking in such mundanities as parking brakes. Suuure, V8 centermounted engines and carbonfibre-chrome-molybdenum exhaust pipes get described in loving detail, but the stuff I’m interested in? Not a word.)
Thnaks for the replies!