It is possible to use both at the same time. It's not possible to use both a Java based AJAX viewer and a PHP based AJAX viewer at the same time unless some creative URL redirection is implemented and even then I'm not sure it's possible. PHP is always installed by default to support Web Admin; however, the PHP API components are not installed. Go back to Add/Remove programs and modify the existing Web Extensions install to add the PHP component. With this setup the Java AJAX viewer will be used, but the PHP API should be available.
Regards,
Dave