Are these captive or wild swans? Do they have a family or are these the only 2 swans on the pond?

If they are wild swans, there isn't much you can do as they will have to work it out.

If they are captive and the only 2 swans on the pond, you can capture both swans and give them a time-out in a garage, shed or other room/facility where predators cannot get to them.

This time of the year is mating season and the testosterone levels will be decreasing shortly and the fighting should calm down. If these are the only swans on the pond and have been raised together, they are not likely to kill each other. More than likely, just a little brotherly spat, but it can result in injuries, maybe serious injuries. You might ask why take them both to an indoor facility? If you leave one on the pond, he is going to really own the pond, as his own. Same thing occurs if you leave one on the pond and put one in a pen on the pond. By bringing them both indoors at the same time and keeping them together, it is a strange place and no one gets to play king of the mountain. This gives the hormones and attitudes a time to calm.

If they have a family, this not going to work and you may have to find a way to separate the pond so one side belongs to one and vice versa and they can't contact each other. Capturing these large birds, especially at this time of the year is not going to be easy and you could get hurt. So, if capture and separation is not possible, you will need to chase, intervene or weather the storm and hope no one gets hurt. Should one of them get seriously hurt, you will need to get him to a vet and find him another home. Do not place him back on the pond.

This situation is exactly why we do not recommend Trumpeter Swans in this type captive setting because they are much larger and more dangerous than the smaller Mute Swans. But, wildlife officials want to get rid of the Mutes so they can introduce the larger Trumpeters for Trophy Waterfowl hunting purposes.
In doing so, they are promoting Trumpeter Swans even in captive settings and this is the problem that is going to continue occurring until wildlife officials are told enough is enough. The Regal Swan