[Zope] Switching users with loginmanager

I am having difficulties switching users with loginmanager. I have an entry page, which is lnked from an external site. The other site sets __ac_user and __ac_password before linking (through a form with method=POST). If my user follows the link, he can access the page without problems. The problem is that if he changes user in the external site and follows the link again, __ac_user is ignored, since the user is already authenticated, and from the Zope point of view it is still the same old user. Any clues about how to force re-authentication in that page? By the way, redirect will not work because I need some other fields in the POST, and I would not want them to be shown in the URL. Thanks, Sergio