SSO with redirected challenge

Hi friends. I'm having a bit of an issue here with SSO.

We have a SSO configuration that works just fine with Portal Access' that authenticate "directly" - the same site performs the challenge. However, we have another portal that, redirects you to another URL, and that site is the one that performs the authentication. We have added both sites as SSO Domains, and both urls as resources in the portal access object, but to no avail.

Thank you for your help!