That's a very good question! I co-write every day here in Nashville and the rule of thumb is that the song is divided by the number of writers... 2 writers equals 50/50... But just because that is customary doesn't mean it is written in stone. I don't know of a specific .doc file that we have for an agreement between two writers except for an agreement we have concerning the mechanical rate in the case of one writer being subject to a 'controlled compososition' clause in his/her recording...