I’ve been asked a number of times about this, and the answer is quite simple — though frustratingly hard to find.
The way to do it is through an application with the confusing name of Audio MIDI Setup (found in your Mac’s Applications ? Utilities folder). This program can be used to combine any audio inputs on your computer into one “Super Input” (called an Aggregate Device be Apple) that can be used in any program.
The cool thing about this is that you can assign these inputs to be different channels of the Aggregate Device, so you still get the same control in Garageband that you would with a full set of inputs on a device like a firewire mixer. You can also use this Aggregate Device in iChat or Skype for multi-way chat — just select it as the audio input in the program’s preferences.
Here are the specific directions from the Audio MIDI Setup help file:
To combine audio devices:
- Open Audio Midi Setup in the Applications/Utilities folder.
- Choose Audio > Open Aggregate Device Editor.
- In the editor window, click the Add (+) button to create an aggregate device. You can select the device and rename it.
- In the Aggregate Device Structure list, click the Use checkbox of each device you want to include in the aggregate device. The list shows the currently connected audio devices and the number of input and output channels for each.
- To set the clock of one device as the master clock for all the combined devices, click the Clock radio button beside the device name.
- After selecting the devices you want combined, click Done.
There are some more options to play around with in the program, but that should get you headed in the right direction.