Goes way back.
When Football was a game mostly played on a Saturday afternoon with a three o'clock kick off, we would alternate home games.
So the Northumberland mining villages would come to watch us every other week then N/C.
Their coaches would park up at Roker.
This was a 'two way thing as when N/C were at home the coaches from the Durham mining villages would be parked up outside of Sid James.
For about half of my lifetime it has been not only possible, but quite acceptable to support BOTH Clubs.
And when it really matters I think you will find that most supporters of both Clubs still do.