apache2 and phpbb2

[Marc <debian@roth.lu>]

Hi,
I currently have 2 virtual hosts that are supposed to use one
installation of phpBB. I have used the prepend feature as it is
suggested in the documentation.
What I get is very strange: each one of the two virtual hosts selects
one of the phpBBs at random... :( I have even tried different IPs, so
the problem is not name virtual hosts alone.

In summary, the apache2 config looks like:
.
.
.
Alias /phpbb/images/avatars /var/lib/phpbb2/avatars
Alias /phpbb /usr/share/phpbb2/site
<DirectoryMatch /usr/share/phpbb2/site/>
.
.
</DirectoryMatch>
.
.
.
<Virtualhost 1.1.1.1>
..
php_value auto_prepend_file /home/..../phpbb2_web1_config.php
ServerName web1:80
..
</VirtualHost>
<VirtualHost 2.2.2.2>
..
php_value auto_prepend_file /home/.../phpbb2_web2_config.php
ServerName web2:80
..
</VirtualHost>
.
.
.

What could be the problem here?
Regards,
Marc