I am not sure why there are two. Ideally there would not be.
For my clients, we setup two maildirs, one for bounce-* emails and one for notification-* emails.
The acs-mail-lite sweeper checks the bounce Maildir and notifications checks the notificaiton Maildir.
If you have both packages configured to use the same Maildir, one or the other will sweep and delete all the email intended for the other package.
