As a solution I can propose to install SubGit into both repositories --- it will create a Git interface for SVN repository and SVN interface for Git. Then you can add svn:externals to the main repository for SVN interface and add a submodule to its (the same main repository's) Git, interface using SVN and Git interfaces of your slave repository.
To install SubGit for Git repository run:
$ svnadmin create svn.repository
$ subgit configure svn.repository
$ #edit svn.repository/conf/subgit.conf ('git.default.repository' option)
to set path to your bare Git repository (the repository you on the server)
$ #optionally prepare svn.repository/conf/authors.txt file
to configure custom authors mapping (if you don't like automatic default mapping)
$ subgit install svn.repository
To install SubGit into SVN repository, perform the same steps, but use existing SVN path instead of svn.repository and non-existing path for git.default.repository (a Git repository will be created at this path).
SubGit will keep in sync both interfaces of every repository, both interfaces are writable.
Hope, this helps.