Should co-first authors be listed in alphabetical order?
I don't see any way to view this other than as a misunderstanding or miscommunication.  (Not necessarily an innocuous misunderstanding: perhaps someone is trying to pull a fast one of some sort.)   

I am a mathematician, so maybe I am especially sensitive to logical issues, but I can't think of a situation in which someone tells me something that sounds like a logical contradiction in which I wouldn't just say, "I'm sorry: I must be confused because that sounds contradictory.  Could you please explain it again?" 

My only guess at the resolution of the contradiction is that someone is in fact trying to change their mind about the ordering of the authors.  I know little about the conventions and nuances of author ordering (because in pure mathematics the order is almost always strictly alphabetical), but I do think I understand the meanings of all the terms involved, so I can only imagine that if you submit a paper to a journal saying "by Bravo, Alpha, Charlie....Please note that Bravo and Alpha are co-first authors" then the editors will respond by calling attention to the contradiction.  What else?