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.