1) Install appropriate driver for the operating system
2) Connect a word clock cable from the worcdclock out of unit #1 to the wordclock in of unit #2. Alternatively, if you use a master clock, you can connect wordclock cables to the inputs of both units
3) With Thunderbolt, use NControl or the front panel controls to set Sync Source for unit #2 to "Word clock" (or both units to Word clock if using a master clock). For USB, disconnect the USB cable and set the clocks as described above with the front panel controls. Reconnect USB when finished.
4) Open Audio Midi setup from Applications > Utilities
5) Click the "+" in the bottom left corner and select "Create aggregate device"
6) Select Aurora #1 first, and then Aurora #2. De-Select "drift" from Aurora #2
7) Rename the aggregate device if you wish
8) In your DAW, select the aggregate device as your audio device