@ eschwartz : you were almost answering my question while i was writing it (see my last post ...)
If I understood correctly, I think that it would solve my problem if i use the option you propose to have only the fist author as I would have only one folder for author A in it's parent folder "A" :
{author_sort[0]:list_item(0,&)}/{author_sort:list_item(0,&)}/{title}/{title} - {authors:list_item(0,&)}
But as I mentioned in my last post, a new question came to my mind ....
When I will have backed up my library on my cloud ... will i be able to search in my Cloud by the 2 authors names ? I mean will it possible for me to search all the books of author A and being able to search for all all the books of author X (if author X have also written other stand alone books for example ... if it does not create a subfolder in the parent folder X for the book author X has written with author A, if I'm looking for all the books author X has written and i don't remember about the book he has co-written with author A, will it show it to me if I search for "author X" in the Cloud for example ?)
What do you think about this ? Maybe my question is not appropriate or maybe it's a silly one but I've just discovered the whole "cloud" concept and I'm really not familiar with it's utilisation so I'm not quite sure if I'll be doing everything the right way ...