Windows will only allow one domain per server instance. To install another domain, you'll either need another physical box or you'll need to virtualize the second instance of Windows Server. Either way, you'll need a second license for your server software.
If money is tight, you could install VMware Server (free) on your existing Windows Server and install a second instance within a VM on the same physical hardware. This certainly is not a workable long-term solution, but for testing/development purposes, it should be just fine.
Regarding your reason for needing two domains, it's not immediately clear why you'd need two domains. Can you expound on that?