Ok so this was not a clever question. We found that you can actually set two externals on the same base root directory, just with a different local paths.
So in Tortoise SVN, you can add two externals with different but nested paths on the same root directory. I haven't done this with the command line but it should be possible as well.
After this the properties are:
externs$ svn propget svn:externals .
http://test.example.com/project/library1/trunk library1
http://test.example.com/project/library2/trunk library1/library2