So, at home, I have a couple of double network sockets … cheap things but they work.
In my attic I wired one up and, having only one wire in, figured I could simply run wires from one socket to the other (duplicate configuration i.e. port 1 connected to port 1, 2 to 2 etc.) and so provide two working sockets. Each socket works AOK but clearly I’ve done something wrong since both won’t work at the same time so either I need to do some kind of in-socket-crossover or I need two wires in.