I can see some potential difficulties with this concept but the idea is:
I change a file in Repository A and commit, Repository B has a file that is a link to a fil
Check out this post..
It helped me
how to properly set svn svn externals property in svn command line
For those who dont want to follow the link here is a brief overview.
svn propset svn:externals 'akismet http://plugins.svn.wordpress.org/akismet/trunk' .
After running the above do a
svn commit
and
svn update