externals and shared files/directories

I hope someone can help.

I take part in a small group that is developing several projects and we were
planning to use Subversion before the number of source code files will be
unmanageable. One of them projects contains core functionality as well as
customizable configuration files that are shared among the other projects.
The other projects are incorporated like plugins. Imagine the following
structure:

/core
/project1
/project2

I was planning on creating these projects in Subversion and use
svn:externals in order to get the core functionality. It seems easy if a new
project is defined where it has both core and project X as external
references. For example:
/core svn://server/core
/core/project1 svn://server/project1

However, the above mentioned configuration files are located within the tree
structure of the core project. Firstly, I tried to set in the same directory
core and project1, but Subversion doesn't allow that. Has anybody any idea
of how to deal with this??

Thanks,

-- 
Dario.