We cannot have multiple session-config elements in the web.xml.
To clarify the difference between 0 and -1 values for sesion-timeouts in web.xml, here is an extract from the specs.
Servlet 2.4 specs, p:141
12. session-config Element
The session-config defines the session parameters for this Web application. The sub-element session-timeout defines the default session timeout interval for all sessions created in this Web application. The specified timeout must be expressed in a whole number of minutes. If the timeout is 0 or less, the container ensures the default behaviour of sessions is never to time out. If this element is not specified, the container must set its default timeout period.
Either zero or negative, the behaviour of the session-timeout is expected to the same. i.e) The sessions never expire!