Why can’t I share Session state between 2 web apps with StateServer? What am I missing?

I’m having trouble getting 2 identical ASP.NET MVC applications to share the same Session using a Session StateServer. The reason I’m trying to do this is we will eventually be deploying this app across 3 web servers that need to share the same state. We need to use StateServer because we are trying to minimise use of the db for non data-related storage.