Delete EnableSessionState attribute in the @Page directive and make sure it's enabled in web.config, so Session will work in all pages. Then make sure that the new window you open is opened with window.open javascript function and not via A tag like: <a href="whatever_page.aspx" TARGET="_blank"> this way the other window will reside in a different session and it won't work. At least that's the way I think it is.

And of course, make sure both pages are part of the same application. Applications don't share Session object  collection...