Note: Discussion successfully moved from VMware ESXi 5 to Virtual Machine & Guest OS
You can add a second vCPU to a 2008 R2 VM by shutting it down and making those changes. Windows itself will need no additional changes made. With the host processor setup you describe, this will be fine.
Just out of curiosity - What is consuming the CPU? I have many many 2008 R2 single vCPU VMs that have no issues.