Would it do for you to merge your projects by importing the second project as audio file into the first project? Then the following should work:
After finishing the second project, share it to the Media Browser as an Audio File:
From the GarageBand help:
You can share audio files to the Media Browser, where you can view and open them in other Apple applications, including iMovie and Final Cut Pro.
Share an audio file to the Media Browser
Choose Share > Media Browser.
The entire project, from the beginning to the end of the last region, is bounced to an audio file named after the project, and shared to the Media Browser. Any silence at the beginning or end of the project is trimmed. If Cycle mode is on when you share the project, the part of the project between the start and end of the cycle area is bounced and shared.
Then, in your first project, create a new audio track and import the audio from the second track from the Media Browser.