The common element are the ipads. I have a mac and he has an old pc. Is it possible to make this work only using our ipads?
It is not (yet) possible to send editable projects directly from iPad to iPad. You need a detour using the computers.
From your iPads to the computer is easy, see the GarageBand Help:
This way you can download a GarageBand project. Compress it and mail it to your friend. On your mac you can upload the project the same way using iTunes, only on a PC it is more involved:
See turingtest2's post here:
also turingtest2's solution to upload project files from your PC back to the iPad.
Why would Apple not allow tracks to be shared? Is this something they are working on?
We can only speculate. And speculation is frowned upon in these forums.
But Apple has recently added the ability to send GarageBand projects to your mac, and you can continue to work on it on your mac, so that feature is clearly under developement.
I'd send feedback to Apple and let them know, that sharing of projects should be supported.