be careful when using OSX's aggregate audio with multi interfaces w/o being aware of clocking them properly.
**Be sure to make your audio interface the master clock. If you add multiple audio interfaces to your Aggregate Device, you'll have to make sure that one audio interface is sending sync to the other devices (over a wire). If all your digital devices aren't locked to the same clock, their timing will drift very slightly and you'll hear clicks and pops.
If any of your interfaces can't receive sync or if your master interface can't send sync, you have one additional option. Audio MIDI Setup includes a "resample" checkbox for each device. With this checked, Core Audio will resample the audio streams from the device to ensure they are in sync with the other devices.
Once you have added your desired devices to your Aggregate Device and selected one of your devices to be your clock master, double-click on the name "New Aggregate Device" and give it a more appropriate name, as shown in Fig. 3. Now you can choose this device as your Core Audio device and you will have access to all of the inputs and outputs in the Aggregate Device.