Yes, a powerful entity, a god, or a special hybrid can command both werewolves and vampires. [1, 2]

Mythological and Fiction Rules

• Deities and Creators: In many fantasy universes, the original gods or creators who started the curses can control both sides. For example, if two opposing higher powers (like Daedric Princes Hircine for werewolves and Molag Bal for vampires) worked together, their combined power could theoretically rule both. [1]
• Supernatural Hybrids: A rare hybrid being that possesses traits of both species can sometimes exert a dual-command authority, earning respect or fear from both camps due to its shared bloodline. [1]
• Mind Control and Magic: An ancient arch-vampire, a powerful witch, or an elder demon with extreme psychic or magical dominance can sometimes force both creatures to obey, even if the two species naturally hate each other. [1]