Many audio applications only allow a user to select one audio interface even if you have two more more connected, and even if they're the same device. Mac OS 10.4 "Tiger" added the Aggregate Device driver in AudioMIDI Setup to help allow multiple interfaces to work in one application.

An example setup below uses Ableton LIVE and two RME Fireface800 interfaces. The steps below assume your interface and drivers are already installed and operating individually.

1. Open Audio MIDI Setup (from the Applications/Utilities folder) and vist the Audio Devices page.
2. Go to the Audio Menu - Open Aggregate Device Editor
3 . Click on the + button to create a new device
4 It's a good idea to rename it (double click on Aggregate Device in the list and enter the new name, e.g. "BOTH FIREFACES." Be creative so it's easy to identify)
5 The list at the bottom of the window will show all the audio devices connected to the computer. Select the ones you want the device to use by checking the “use” button. In this case, select both FireFace800s.
6. Click "Done" and Quit AudioMIDI setup.

7. Start your music production software, LIVE in this case. In your audio driver setup, the new Aggregate Device should be there as a choice. In this case, LIVE menu > Preferences > Audio... set Input and Output Audio device to the Aggregate Device.