Scaffolding Spring Security across multiple webapps

How can 2 webapps share the same login, from the users point of view?

My solution involves a number of scaffolded webapps. Spring Security is working great on each. They share the same User tables etc.

My hope was that they would share the same security session, so logging in under one would log you in as far as the other is concerned. But they dont.

This reference in Google is the only one I can find, but its 2007 and the links to the examples are broken.

http://springtips.blogspot.com/2007/06/using-shared-parent-application-context.html

Thanks