[exim] Sending Duplicate mail to Multiple Mail servers

We have a client that we relay mail to and they are in the process of
transitioning from a NT server to a Sun server however during the transition
they would like to run the 2 servers side by side for a month or so just in
case they have problems with there new Sun server. How would I relay
duplicate mail to both these servers?

Exim version is 4.23