A few years back at the fair when I was hanging around the ancient-gentlemen-with-their-tractors section (my favorite area), there was an old man with a 1919 Fordson. My brother-in-law and myself stayed and chatted with him for well over an hour about that tractor, and many other that he had used, restored, or owned just to have, during his lifetime. At some point in time during the tales of his tractors, a two-seater was mentioned. I don't remember if he bought it that way, or if he made the addition himself.
I'll have to ask my brother-in-law, and see if he remembers what the explanation was. I want to say it was to have someone operate a piece of equipment while the driver operated the tractor.