With 1 physical server machine (installed with Window Server 2008R2), can I have two domains? For example, testA.com and testB.com. Each of these two domains has its own user groups.
Not as I think you're hoping to have. A single machine can be a domain controller for a single domain.
But, what you can do is use the Hyper-V role on Windows Server 2008 R2, or replace it with another hypervisor (such as VMWare ESXi) and use that to create two virtual machines. Each of these virtual machines can be a domain controller of different domains.
Depending on your virtual network setup, you could create
Depending on the version of Windows you have licensed, you are entitled to a certain number of virtualised instances of the OS. For example, Enterprise edition you get one physical + four virtual.