Haven't read Yorknew arc in a while; fair enough, they wanted to kill Chrollo. Theyre professionals, so it makes sense to do a 2v1 to make sure Chrollo lose. Im not sure if it was stated, but what one thing I got from watching the Zoldycks, they keep tabs on every family member. Meaning they knew of the targets for the day. My headcannon says, Zeno and SIlva knew of Illumi mission as did Illumi, because of how the the zoldycks operate.