The two teams were said to be on good terms during the Second World War, with City allowing United to play their home games at Maine Road due to the damage that had been done at Old Trafford.
According to Manchester football historian Gary James, Frank Johnson, City's vice-chairman at the time, was keen on having the two Manchester giants join forces.
"The idea was killed by both clubs before it ever became public," James explained in an interview with the Manchester Evening News. “I spoke to Eric Alexander whose dad Albert was chairman at the time, and he said Frank Johnson, who came up with the idea, often came up with crazy ideas.
“Another of his plans was to make the entire league regionalised into north and south. But City were at a real low in their history at the time. In terms of league position, it wasn’t as bad as 1998-99, but in terms of general morale, atmosphere and support it was by far the lowest point in the club’s history.
“In the late Nineties, we still had over 30,000 going to games, and that meant the club still had a high profile. In 1964-65 we were in the second division, support had dropped to a low of less than 15,000, and general interest in the club had also dropped.
"There was a feeling that this could happen to any club. In fact, all it needed at City was a plan and a vision, and to bring in the right manager. I always believed in the Nineties that City would come back, because of the strength of the support, but in those days in the Sixties a lot of people didn’t feel that way."