Yes, At least in IIS4 you could, as I don't know what version your are using, and I'm not familiar enough with IIS5 I'll speak about IIS4.
You can have multiple SSL web sites with a single IP using Host header information. This is accessed through the website configuration. You can have multiple websites with one IP address, the only qualification is these each website must have a unique FQDN(fully qualified domain name, i.e. www.microsoft.com), or a unique IP address, or a unique port.
In your case the first applies. Give each SSL Site a different FQDN with the same IP and port number.
As far as the certificates go. the certificate will only be valid for the site that it was issued to....however it will work with the other sites, the users will get an warning message informing them that the secure site that they are going to does not match the certificate...SSL will still work tho.
Yes, you can have several SSL based on a single IP. This must be possible for web hosting company to host several websites on a single machine with a single IP but requires different SSL for different site.
If you're asking for technical help, please be sure to include all your system info, including operating system, model number, and any other specifics related to the problem. Also please exercise your best judgment when posting in the forums--revealing personal information such as your e-mail address, telephone number, and address is not recommended.
Multiple SSL Site on a single IP address
If yes, certificates of which site will be valid