A good noun for a two-faced person

Janus?

In ancient Roman religion and mythology, Janus is the god of beginnings and transitions [...]. Most often he is depicted as having two heads, facing opposite directions: one head looks eastward and the other westward. Symbolically they look simultaneously into the future and the past, back at the last year and forward at the new.

Well, unfortunately, they're usually just called "a two-faced person", and most other words dealing with it are adjectives that in noun form simply refer to the abstract quality of the adjective (a -ness noun, e.g. "deceitfulness") rather than one who has the quality of the adjective. Let's see what I can come up with though.

"Double-dealer" - One who says one thing and does another, or one who does two things which are diametrically opposed to one another (such as selling weapons to both sides in a war, playing both sides against the middle)

"Fake" - usually slang in this context, one who acts one way but thinks something else. Like someone who pretends to be your friend to your face, but talks about you behind your back

"Sociopath" - Sociopaths usually display an overt duality of personality, one which acts as a front to cover for their real (completely unemotional and ruthless) personality. Hence, you get Sociopathic serial killers who live completely normal lives while nobody suspects that they're torturing people to death in their spare time. However, this refers to an actual medical condition, but it's not uncommon to use.

"Traitor" - One who betrays.

"Turncoat" - A traitor who switches sides, principles, etc.

"Deceiver" - One who deceives.

That's all I can think of right now.

You could try duplicitous.

Jekyll and Hyde, maybe? This is very much linked to good/evil, though.