On another knitting forum I saw a post referring to this: http://knitty.com/ISSUEfall06/FEATextreme2in1.html. It is how you knit two socks at the same time using a technique called double knitting (https://knittinghelp.com/video/play/double-knitting).
I do not think this technique is for everyone, as we have different knitting styles. For me it would slow me down so I rather knit the socks separately one after the other. Still I am sure that this technique might be really usable for some visiting this forum. One advantage is that one does not need to do any row counting when producing the other sock, as it is made at the same time.