When you buy or configure your Multi-Domain (SAN) SSL Certificate, please note that most Multi-Domain Certificates do not secure the domains with and without “www”. With other words if you want to secure both, example.com and www.example.com under one single Multi-Domain Certificate, that will be considered as two different domain names. The screenshot on the right shows you where you can find the attribute that tells you if your Multi-Domain Certificate secures both “www” and “non-www” under one single domain (SAN), or not.
Anyway, that is not a problem so as you cannot have the same website open both as www.example.com and as example.com. All website owners only choose one of these options and make the other option automatically re-direct to the other. For example, you can choose your website to always open at www.example.com and anybody who enters on example.com is automatically redirected to www.example.com. In this way, you only have to secure one domain, and that is: www.example.com.