There is a couple of friends of mine that has published some works, and they always work together... They say they find the collaboration helpful while writing dialogues, when they act one character each... They say that often the other came out with thoughts and hints that the words said by a character can be read differently from what the actual writer intended, and that usually this tecnique is helpful to have the dialogues more realistic than the ones written by a single person... They have not a main author who has the last word... At least they state that their work is basically on the same plane... Since there are some years they go that way I think that for them it works... This just to refer some experience... I think that it mostly depends on the relationship you can establish with the other author. To tell the whole truth I must say another thing: I don't like very much their way of writing, but I don't know if this disliking comes from their writing togeher, or if they both have a style I don't like very much... (Just to let you know I'm still alive, and always reading the writer forum, even if my difficulties in writing in English keep me from writing comment or posting something from me...)