Is it really necessary to have a unique IP address for the SSL certificate to work?

Yes. You need to have a unique IP address for each domain you want to secure. This is an SSL protocol thing and has to do with the Secure Sockets Layer working below the application layer. Any site that wants to use an SSL certificate must have its own unique IP address that is not shared by another site. The IP can be real (routable) or internal (RFC 1918 non-routable address) but, it must be unique on the server. Please also note that it doesnt matter if the IP address you assign to the site changes later. If you decide to change the IP address later you wont need to get a new SSL certificate. The SSL certificate must be bound to an IP, but not a specific IP. Your hosting company should be able to set you up with a unique IP address if you dont already have one.