I would stick to one single authorative repository or you'd end up with a version hell at some point.
If you intend to have external developers contributing code on the codeplex side you will need to merge your changes with theirs and also integrate their changes on your own internal TFS server.
It's safer to have one single authoritive repository and just create snapshots for milestone releases on the other.
You could do your fine grained check-ins and modifications on your internal repository and periodically integrate/merge them to the codeplex code-tree. However what works on one codebase may not work so well on the other after integrating, the sooner you integrate changes the better (don't work on your own isolated branch too long).