There are a number of series which are written by more than one author.  Most of these tend to be media related like Star Trek and Warhammer, but there are others as well.  I have attempted here to provide that information here.  Most series are listed in random order or the order in which they were published, but when it seemed useful to group them by some internal chronology, by sub-series, or in some other fashion I have done so. When there is an internal chronology, I have tried to arrange the titles in the proper order.  
  Multi-Author Series - Science Fiction  
  Multi-Author Series - Fantasy  
  Multi-Author Series - Horror