When it comes to why Matthew's account mentions two demoniacs, one could argue that, although the book of Mark was probably written first, Matthew actually witnessed this event. Matthew, therefore, actually interpreted the facts correctly. But Jesus called Matthew in chapter 9, after the healing of the demoniacs. This does not mean that Matthew would not have been personally informed by another disciple of the event. He probably knew the details of the story, but for the sake of his gospel he did not write it down in depth. Another explanation for Matthew's inclusion of two demoniacs would be to demonstrate that Christ healed more than one demoniac. Matthew may have combined this story with another that he did not include (i.e. Mark's first exorcism in 1:23-28). Finally, since Matthew was probably writing his Gospel to the Jews, he included the second person as an additional witness and to fulfill the legal requirements of the time. This is probably not the case for the last two. It seems strange that Matteo combined demonic stories. Furthermore, since there were swine herders and Jesus' disciples, there would have been enough witnesses