I've extend the Garageband tools. There is now a converter from Mac Garageband to:

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So a song can be worked on, on the Mac and on the iPad

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and even iPod 4 or 5

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I've also created and nice showreel showing two songs being written and how easy it is to share Garageband Songs between the Mac, iPad, iPhone or iPod adding tracks as you go. The video is just 1 min : 52 secs long and really shows the creative power of this approach.

Although there are sites showing how to manually edit songs, it is really tedious and error prone, I know I've tried it and gave up and wrote the converters.

Showreel showing songs being created across the Mac , iPad and iPod