Israel Zangwill wrote a century ago that “Life is Janus-faced, and the humorist invests his characters with a double mask; they stand for comedy as well as for tragedy.”
Janus, a Roman God, has the image of a man with two faces, looking forwards and backwards. Read more about him here.
It seems obvious what the phrase means when used today. A person is portraying two distinct personalities, at least one of them deceitful. In case you weren't sure, it's not a compliment.