Two way you can do this,
1. Using your DNS server, created an alia for the domain and assign tot he IIS IP address. Or if you use DNS Round Robing you can assing multiple domain to the same IP address.
2. You can assing a second IP address to the IIS server then assing your domain to this new IP address. You make this change in the Network TCPIP configuration on the server, and the IP address need to be on the same subnet as the original.