Recently when I was doing new RapidSSL setup for www domains, I found out the certificate protect both with and without www.
On the approval email from RapidSSL, it lists both secure URLs www.domainname.com and alt URL domainname.com. It means 1 SSL certificate protects both URLs.
However, it needs to be confirmed. I am still looking for official announcement and confirmation from RapidSSL. I will be updating here. Stay tune