This is because a SSL certificate is tied to the exact domain name used in the URL. If you use www.yourdomain.com for the SSL certificate, it will only work if the URL that is requested is www.yourdomain.com. It will not work if it is called yourdomain.com or www2.yourdomain.com. If you also wish to have people access the site securely using a second hostname/domain name combination, you must purchase a second SSL certificate.
