It should be possible to union mount annexes. So if multiple drives have
content, an annex mounting them both would have available all the 
content from all the drives.

This could be done by just making .git/annex/KEY link to the actual content
on the mounted annex.

(Need to make sure the [[copy_tracking|copies]] code does not
confused and think the symlink is a copy of the content.. Also need to make
sure that code that writes to .git/annex does not follow symlinks.))
