Force SSL using different domain

Hello,

In the wp-config file i have added define('FORCE_SSL_LOGIN', true); to force the use of SSL when logging in. I am running WP on http://a.com but don’t have a valid SSL certificate for it. I do have a valid certificate for http://b.com which points to exactly the same server etc. The WP installation works fine on both domains (but obviously reverts to a.com when you click on any internal links).

I have changed the value of uri_ssl in the db so that it uses https://b.com. When i log in it seems to authenticate and then returns to a.com but the session is not active. I am guessing this is because it is being authenticated against a different domain and this does something funny with the cookie?

Any ideas on how i can get this to work?

Thanks, Michael