I have two on my Ducati and only one on my CB. Basically, it was an, "Okay, right now I can only afford one, the law only requires one... one it is..." Another shall soon be bought... probably.
As for the throttle, you just have to mount it carefully. On the left hand side, you want the mirror placed as far in as you can get it (instinctually), but on the throttle side, leave it out just far enough that the mirror housing doesn't touch the twisting part of the throttle. otherwise, it might cause it to bind a little bit, or slow down in it's snap bac to closed.