Ran across this issue the other day when trying to get multiple local sites running in IIS with self-signed certificates: You can only have 1 site bound to the IP address of the server. So if you only have 1 IP address on that server you can only have 1 Self-Signed Cert running.
BUT there is a way to get multiple SSL sites running off a wildcard Self-Signed Cert though. Follow the steps listed on this site:
But you will need the old SelfSSL.exe and not the one that comes with IIS 7/7.5. Get it here -
Scroll to the bottom and read where you can get it. I've also linked it directly for convenience (if it changes it could break though).
I was able to get multiple sites running with the wildcard SSL and using host headers for the sites.