Until recently it has been impossible to install multiple SSL certificates for virtual hosts that are hosted on a single IP address. This has changed with a technology called Server Name Indication (SNI).
SNI passes the Hostname that is being connected to during the handshake process allowing the web server to encrypt the content using the correct SSL certificate. More and more companies are using SNI now and you will need to ask your hosting provider if it is specifically available to you at this time.
Wikipedia has a list of server operating systems and corresponding applications that support SNI.