In general terms, it isn't possible to get an SSL certificate that is *guaranteed* to work for more than one domain, when listening on the same IP and same port.
There are routes, but most of these aren't guaranteed to work, require multiple ports, require all the domains to share a common root, or some combination of the above.
Ok, from the top then.
first option is to give each SSL site on the shared IP its own port. So instead of using 443 (the default) you assign 444, 445 etc. The urls then look like
https://<name of host>:<nonstandard port>/
which is a pain to give someone, but of course all the http sites can still share port 80, so provided your https use model doesn't require that users are able to just type
https://<name of host>/ and get straight there (but click a link or a bookmark) it works ok. You might also find that some web proxies will not permit connections to other than 443 (anti evasion measure) so some users are going to have issues.
Second option, wildcard certificates.
These are only usable if all the sites share the format
https://<name of server>.<your domain>/ so that a certificate of the format *.<your domain> will match on the * for all your sites.
Third option, microsoft style multi-host certificates
These are called Subject Alternative Name (SAN) certificates, and are increasingly supported in modern browsers:
http://www.digicert.com/subject-alternative-name.htmdownside is that they aren't supported in ALL browsers, and in fact some will flag it as a security violation, ignore all but the primary (non SAN) name, or both.
Final option: get separate IPs on your hosting.
This is often cheaper than SAN or wildcard certificates (for which CAs charge a LOT) and some hosting centers bundle the first 5 IPs with the basic package (and are quite reasonable for further allocations) - Certainly the budget provider I use (10ukp/month) gives 5 IPs in the basic vhost package.