If male swans grow up together at a very young age, they will bond and form a pair. Therefore, they do not need to be separated unless they constantly fight and one is refused access to the water and habitat.

Because they are older and have grown up together, but alone on their side of the pond, it is highly unlikely either will accept another swan in their habitat. For the safety of the female swans and your sanity, it would be better to return the female swans from where they were procured. If this is not possible, you need to find them another safe home. The Regal Swan