I´m going on this image: http://www.marechal.fr/statiques/upload/docs/pdf/CAHIER-COTE... (p.24)
where they avoid the whole naming problem by simply calling it a connector. Something along the lines of simply a "coupler" or a "latch-on connector" might be more desirable, but I can imagine native speakers might object to that term, and it googles even more abominably than koppelcontactdoos.
Which brings on the second problem: I don´t have the faintest idea what they mean; whether it is a socket, a combination of socket and plug, two sockets, and what kind of socket anyway....
At least this might provide some inspiration :)