Splitting a stereo audio into 2 mono tracks in Garageband

Can it be done?

YES

Step by step:

1. With your stereo audio in Garageband first pan the audio to the left.
2. Export the audio. We will call the exported audio: LEFT MONO.
3. Close GarageBand and start a new empty project.
4. Import LEFT MONO into GarageBand.
5. Inside GarageBand click the Smart Control button.
6. Under the Smart Control panel go to Control tab.
7. Under Control tab, in the left side check the "input" section and click the button to switch between stereo and mono input. Since your audio is stereo, the default value should be stereo. So switch the state to mono. Now you should be able to hear your audio in mono.
8. Now export your audio and there you have it, the left side of the original audio in mono.
9. Repeat same process with the right side of the audio.