Connect them to two adjacent sockets in the same room (so the same circuit) and best with nothing else attached (on at least on) for the test period. Attach a computer to each and run LANSpeedTest or do timed transfers of some large files. Also, make sure neither is on a surge suppressor. If your devices are 200/500Mbps you will get 40-100Mbps if they work well at that proximity. If you cross buses in the breaker box speeds can slow dramatically.
I tried this both via LST and file copy, however it denies write access to the other PC. I set up all references and I can explore both computers from each side. How do I fix this? I tried running LST as admin but that didn't work...
If both are Windows 7 you need to change two things on each machine to allow bidirectional access -- sharing and security settings.
For each drive: first select a folder or drive to share, in Windows Explorer right click on the file and select properties. Under the security, click edit, add, type Everyone in the box and then allow that group to read and write by checking the boxes. Apply, then OK. Then open the Sharing tab and select Everyone, and choose read/write and OK. Now both machines should be able to read and write to the folder or drive you allowed. Now you can run LST.