They are a father and son pair. Even though there may be some recognition between them, they would still see each other as rivals. As they are solitary creatures anyway, Amur leopards should only be brought together for breeding.
Unsure what the plans are for a female. Maybe when the older male dies, or if the younger male receives a breeding recommendation elsewhere, one can be brought in?
Apparently they are trying to re-locate one of the males and get a female. However there is a Europe wide shortage of female Amur Leopards hence they might be in for a long wait....