I only have to buy one, if I buy any at all, I already have what I want on the rear. Even then I might just do what I did on my last tour and move the brales from my MTB, or whatever is staying behind.
How do they attach? As I recall my old Peugeot, they attached by a single bolt where the fenders attach. In the Paul text they mention how they swore they were never going to use a single bolt, but have relented. So if they weren't center mounted on the crown, where would they be mounted?