They make a Brembo caliper to suit, though It's not as easy as that (it never is!!). BMW installed brakes on top of the forks on some models, underneath on others, so if you get a caliper (available on eBay occasionally) you will have to ensure it will be in the correct orientation. Do you have the mounts for the brake on the other fork leg? If not you will have to get one. I have them already on my Mono, and I even bought a really good caliper of the correct orientation off eBay for AU$10 with the intention of having a twin setup. That was two years ago!!!
What put me off was the cost, I would need two identical rotors, the master cylinder would need to be changed for twin use, and new plumbing. As well, I realised the single disc was still a damn decent stopper, so why just add to the unsprung weight of the front suspension. You have to ask yourself, why do you feel the need, fair enough if you can get it all off a wrecked bike (LS??) for a good price, but otherwise it could be an expensive exercise.
Bill..................;-)