anotherthing, a version of it still is part of the selection process in Germany.
However, its aim is not to follow two "conversations" in detail, but listen to one (which is announced beforehand as the important one) without getting distracted by the other (the "conversations" are actually just letters and numbers, and you have to pick out certain combinations on the "correct" side).
I haven't started my training yet, but would guess this to be quite important for controllers, as you are not exactly sitting all alone in that control room, with other people talking providing a distraction.
Another matter is that people can be able to follow two conversations not in detail, but listen for certain cues which need attention - I did that successfully quite a few times in the Navy, but am not sure if it is applicable for an ATCO.
Regards,
Robert