I don't think you can save directly unless you are running a windows based server (I would be happy to be in error
) I think you are going to have to use an FTP program to get the file to modify then ftp the saved file back to the server. Some FTP programs let you create custom commands to automate a lot of file movement and naming. If you use a naming scheme like I mentioned, you won't have the problem of both of you updating the file at the same time.
Perhaps someone else can come up with a better idea.