First of all, good question!! It took me forever to figure that out!!
Anyway, I looked at that bit, and you are right. It IS two metals! The thing about that one, though, is the fact that there is only one metal in the mouthpiece, which is what really counts. The rules, though not specifically stated, actually mean two metals in the horse's mouth, not in the overall bit! So this bit is legal, since the only metal that will be in the horse's mouth is copper.
The reason they don't allow two metals in the mouthpiece is because it encourages the horse to salivate. While you do want that, since it means the horse has a soft mouth, according to the rules the rider's softening of the hands should do that, not the bit.
I hope I didn't confuse you, sorry if I did :rotfl: