[Exim] Mirroring Incoming Mail in Exim & Cyrus

I'm currently implementing a new mail system for my department, and have
decided to use Cyrus, as I have had very good luck with it in the past.
However, I have one odd requirement which I am attempting to figure out.
I would like a mail message to come in (for instance, to
whelan@???), be processed by the MTA (I am currently
considering Exim and Postfix), then be delivered *twice* to Cyrus. The
first copy will be depositied in the users inbox (for instance,
mail.user.whelan). The second will be delivered into a seperate inbox
(for instance, mail.backup.whelan), which will be moved off the server
and archived on a nightly basis. So, is there any way that this can be
done within Cyrus or within the MTA? Any suggestions are welcome.

Daniel