2012 R2 Std gives you license for 2 virtual servers on ONE physical server (physical server can have up to 2 processor sockets). You will need to buy 1, 2012 R2 Std license for each of your physical servers. If you add any more virtual server 2012 R2 servers to the mix you'll need to buy more licenses.
Microsoft doesn't care if the VM's will only exist on the other server for 5 minutes over the next 2 years. If they ever go live on that server you need to purchase a license to be compliant with Microsoft licensing - period.
Edit - Microsoft has a few confusing licensing articles floating around but I usually just have the vendor I'm dealing with get their MS guy on the line so I can throw scenarios at him and have him explain the depth and breadth of Microsoft's greed.