No you can’t. It has security implications. A bad webapp can act like a
virus, damaging session  info of other webapps (iirc you could do it in
servlet 1.x spec). But I think some app  servers support it by creating
a
single http session for both web  apps. For example you can create a
single session between a webapp running in Lotus Domino and another one
running in Websphere. I think it should be possible to get that
behavior
from other servers too. Check the docs of your server….

Tomcat has two webapps that come with its package. They're named
"admin" and "manager" and allow you to start/stop other webapps and see
their session  information as well. Maybe this could be a start point.
But be very careful in doing this. You may even open a security hole in
your system.