I'm thinking... Import my video of me playing guitar and singing, into Garageband. And then add new Garageband audio track with my backing vocal harmony.
Will this work?
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If you're singing on the new track(s) you've added, definitely! I've done it myself. You can even pan the new track(s) left or right to give your song more "space".
If you were thinking you'd duplicate your audio track in Garageband, and change the pitch of the new track, it's harder. Your harmonies probably won't stay the same number of half steps away from the main melody, so you'll have to adjust the pitch change on most notes. (As far as I know.)