Here's one example of how you can make use of both CPUs:
- Install a free bare-metal hypervisor (VMware vSphere Hypervisor, MS Hyper-V Server, or Linux running KVM for example).
- Create a VM and install your Windows 8.1.
- Create one or more other VM's, and install a free OS like Linux.
- Run more than one VM at a time.
- Do whatever you want in which ever VM(s) you'd like to put load on. :)
The hypervisor will use both CPUs to provide resources for the VMs.