[exim] How to queue_only AND copy mail to a mailbox?

Hello list!

My configuration: Two Exim instances on Debian Woody for a MailScanner
installation. The first exim, called exim_incoming, has only the purpose
to accept mails and put them into the queue (option: queue_only = true).
Then MailScanner takes the mails from the queue, process them and put
them to another queue, the input queue for the second exim, called
exim_outgoing. This exim instance then delivers the mails.
So my question is: How can i copy all mail to a mailbox (for testing
purpose only) and still only queue them? All that should do the first
exim, exim_incoming. Should I use an ACL or a system filter? My idea:
First use the unseen option (like in FAQ Q5033) and copy the mails to a
mailbox, second use an ACL to accept all messages and queue them only.
How to do so? Is that a good/right solution?

A lot of THANKS for any help!
Greetings,
Andreas