Secure Web.Show_Document() calls to Oracle Reports - how to change user credentials on the fly

I am using the procedure outlined in the Oracle Whitepaper listed in the subject line.  It was written by Frank Nimphius and is located at URL: http://www.oracle.com/technology/products/forms/pdf/10g/frmwebshowdoc_rep.pdf  .

The process, as documented, works as long as you are only using one user credential.  The credential is stored in a session cookie and read by the Reports Server to execute the report.

However, if you try to run a second report using a different user credential, the Report Server runs the report using the original user credentials stored in the cookie. The logging shows the cookie is being updated with the new user credentials, but the report is always run with the original credentials.