I'm not aware of such a thing possible. Why do you need this architecture ?
If you need to share state between 2 web apps why not store it in the database ?
As far as I know EJB stateful beans would be a candidate to your problem but the EJB stateful bean is bound to your web session (thanks to Seam). I dont think you can share a web session between different web apps, unless it is the same webapp deployed in a cluster configuration.