If you are able to get more than one IP address added to the virtualhost, then you can use two SSL virtualhosts, with each using one of two SSL certificates. The virtualhosts would each need to identify which IP address they should be using e.g. in Apache Httpd:
This might be cheaper than getting a wildcard certificate, but a wildcard certificate if you're willing to pay for, is the easiest to set up.
SNI (Server Name Indication) isn't supported on all browsers, so although this would allow you to serve multiple certificates using only one IP address, if they both need to be accessible in these browsers, it's not suitable. If only www.domain.com only needed to support older browsers, then you could still do this, as if the www.domain.com SSL certificate is the first one loaded, then users without SNI browser support can still get that certificate.
SNI in Apache Httpd only works on some more recent versions, e.g. not available in CentOS 5.
UCC Certificates are beyond your needs, and are usually very expensive, so I'd not recommend that over wildcard certificates.