i have two web services. One with user functionality, one with admin functionality.
Both services effectively work with the same object types, for instance:
There is an easy way to share types between client and service, just by adding reference to shared type assembly to your client BEFORE adding the service reference.
You can find the detailed scenario and sample project there:
http://blog.walteralmeida.com/2010/08/wcf-tips-and-tricks-share-types-between-server-and-client.html