Onion sets are not pairs of onions but the term given to a bulb that you stick in the ground. No clue where the name came from - I've known them as sets for 50 years!
Much to my surprise, I do have what seems to be sets, as in multiple plants, growing. Last year, son number one for some inexplicable reason mowed my onions, real close to the ground while they were still growing. I harvested what I thought was all of them, but yesterday I discovered two onions growing, but there seems to be three or more stems per location.
Confused yet?
Mike