You want both people to be working with the same file at the same time? The only way I can think of to do this is to use something like Microsoft live meeting or Webex and view the same file together and hand control over to the other user back and forth.
It is slow however.
You can just save the file. Send it to the other user. He works on it and saves a newer version which he sends to you and you open it and look at the differences. For that matter you can import the original version into the new improved file and compare them side by side.