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.