Imo there's no reason for a collab to go slower than back and forth. Its the exact same thing unless you're both on at the same time, in which case it can go by much faster. Also reading a duel with other posts mixed in between would be irritating to me because I'd probably constantly forget the field state, I'd much prefer to read it in one go. However it is GM decision in the end so whatever flys with that is all good.