On a Macbook, GarageBand can do more with the Focusrite 2i2 than Audacity can. 

The 2nd Gen 2i2 should work with your Macbook as soon as it is plugged in, but the 3rd Gen 2i2 may need a driver update.  If it does not download automatically, you can find it here.  Plug the 2i2 in first, and then open GarageBand.  

There's two basic several ways you can use Garageband with the 2i2s, but below are two basic options. 

Both Microphones on a Single Stereo Track

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Next, you choose how GarageBand handles the audio input it gets from the 2i2.  Using the Input dropdown menu on the Choose a track type screen, choose whether you wish Garageband to record a single speaker or both speakers in the first track.  The simplest configuration is to choose a single stereo track for both inputs, by clicking Input 1+2.  This means one microphone will be heard in the lefthand speaker, the other microphone will be heard in the righthand speaker.  This may be suitable for many projects.

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One Microphone Per Track

In GarageBand, you can have each microphone on its own track through the 2i2.  This requires several more steps, but may be worth it if you prefer both microphones to be heard in both speakers, or you would like to subsequently adjust the volume on each microphone within GarageBand.

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