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….