okay, disclaimer; i thought i'd read this on the skill description on AS, but after checking it again, i dont know where exactly i picked this tidbit of info up, so correct me if i'm wrong: it's not necessarily the two middle ones all the time that carry the effects. if you shoot an odd number of arrows it will only be the very middle one arrow that does the "procing." if you fire an even number of arrows, then it'll be the middle two.