[ALUG] Is there a fairly easy way to allow two users to write to the same hierarchy of files?

I want to enable two users to write to a specific hiearchy of files and I really can't see an easy way to do it. To be specific I have a hierarchy of files making up the data files of a wiki which I (mostly) edit directly but also need to be able to edit in the normal wiki fashion. Thus they need to be writeable by the apache2 process (which is user www-data) and directly by the user (which is me, user chris). Can anyone suggest a way to allow these to be writeable by both users? When a user creates a new file it must be editable by the other user as well so simply creating a common group to which both users belong won't work as, in general, the umask won't give files user write permission. -- Chris Green