CAS to CAS Proxy and single SSL cert

Here is the problem - I have Exchange servers with HT/CAS/MAILBOX installed in a "CROSS-SITE" deployment, Site A is internet facing and Site B is CAS proxing against Site A.

The outlook 2007 users in SiteB receive certificate errors as we have a single SSL cert on the Exchange server (mail.company.com) this is also our SCP for autodiscover. If i change the internal URL in Site B from servername.local to use the SSL name (mail.company.com) this fixes the certficate errors.

Unfortunatley it breaks free/busy look up for those users who are based in Site B using outlook 2007.

My question is: Is this a problem with my autodiscover settings or do i need a SAN cert with the server name in SITE B for Free/busy to work?

Thanks