You "access" GarageBand, the artist lesson or any other apps or files because they are "installed" on a specific machine.
You have installed GarageBand on both machine but it seems that you have not installed (downloaded) all the lessons on both machines.
On whatever machine you want to use the artist lessons you have to open GarageBand and download the lessons on that machine. GarageBand stores those files in a specific location. You can actually check if they are there. They are in the system Library. Here is the path:Hope that helps
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