This article describes the process of creating an 'Aggregate Device' with the Audio MIDI Setup utility in Mac OS X computers.
Creating an aggregate device is a way to group multiple audio interfaces into one virtual device so that an audio application can speak to a single, solitary vitual audio interface.
Certain audio software cannot interface to multiple audio device drivers. In these cases, it is necessary to create an aggregate device if you wish to use multiple interfaces. For example, interfacing with Logic simultaneously via a device using optical and another device using USB requires an aggregate device to be created encompassing these two devices. This new group of devices appears as a single aggregate device to the audio application.
Open Audio MIDI Setup
This new aggregate device should now be seen by the audio application.