[Exim] Delivering mail to two systems

I have a working Exim v3 system. I'm in the process of setting up a new
system, which has Exim v4 on it. I'd like to have the two run in parallel
for a while, to make sure I've got good configurations on the new machine for
Exim, as well as other mail related software (SpamAssassin, etc.)

The ideal solution for me would be a way to tell Exim v3 "in addition to what
you normally do, also forward the mail message to this IP:port via SMTP".
Is that doable?

Thanks
David Corbin