Shared SSL is a means of secure Web server access without requiring users to purchase a digital key. You can set it in plesk panel.
In this case, only one domain should have real SSL certificate. We will call it Master SSL Domain. Other domains, which are set up to use Shared SSL will use its resources. To choose the Master SSL Domain from the list of domains which have SSL support enabled, go to the Server > Shared SSL page. In order to do this, set the Enable shared SSL check box and choose, which domain you want to be the Master SSL Domain.
Please refer to the following link for more details:
http://download1.parallels.com/Plesk...uide/index.htm
Serving Your Customers >> Securing Web Sites >> Using SSL Certificate Shared By Another Domain (Shared SSL).