Perhaps turingtest2's fix works for you to transfer your project to iTunes for Windows, see:
But if you want to transfer between two iPads, you could use iCloud to transfer directly between the devices, if the iPads support iCloud and you are using the same Apple ID on both iPads.
If you save the project (or better a duplicate of the project) to iCloud, you will be able to import it on the second iPad from the cloud.
There is an issue that should be solved by Apple. It is not acceptable that the import/export functionality doesn't work under itunes for windows, or that you have to rely on third party software.
That is not professional. The suggested solution to export the audio file in the knowledge base doesn't solve the problem, because I want to share the garageband project. This is a long standing (> 6 months) problem.
I don't understand that Apple didn't use the opportunity of the garageband update or the last itunes update to finally solve this problem, or at least releases a comment that really helps (or announces the problem to be fixed, indicating a date).