how do I send a signal to an external processor?

Hi

In my logic setup I am using an apogee duet for AD conversion.

I want to send a vocal track to a hardware processor that is

connected to my mac via a optical digital connection.

The problem is I only have the choice of output one or two

on the duet. The only solution I could think of is to go into

audio preferences and select digital in and out as my preference

however than I can't use the Duet to hear the sound. In other

words is it an either or situation? Is there a way to use both

the built in digital in and outs on my macpro desktop and

an apogee duet?