Yokozuna is the top rank in sumo but it is not a title for only one. There can be several Yokozuna or none. One becomes a Yokozuna by their overall performance during the tournaments. On the last day of the tournament, if there are two Yokozuna they will have a bout. This is difficult to see in person because the last day is the most popular and is sold out in advance.
A rare chance to see such a bout is at Shinto shrines when they do an outdoor event. Here two Yokozuna square off.