Dfyjkt

In Arabic, there's a handy expression that literally translates as

A mirror to your face, a prickle to your nape (back).

It describes a two-faced friend who is so nice to your face that you'd see yourself in him, but behind your back would say all kinds of bad, defamatory stuff about you.

The Arabic expression is always used as a standalone remark. That is, without being connected to a subject.

Husband: I couldn't believe Jack would say any of those things about me. I was shocked when I overheard him say I was mean and insecure.

Wife: A mirror to your face, a prickle to your nape.

Since you seem to be looking for a phrase, I think this quote from Shakespeare would do nicely:

One may smile, and smile, and be a villain.

The phrase comes from Act 1, Scene 5 of Hamlet. In this scene, Hamlet discovers that Claudius, his seemingly-friendly uncle (and stepfather), murdered Hamlet's father to seize his throne (and Hamlet's mother). On learning this, he describes Claudius like so:

O villain, villain, smiling, damnèd villain!

My tables¹—meet² it is I set it down

That one may smile, and smile, and be a villain—

^{1tables: writing tablets
2meet: fitting, proper}

You can see how the phrase fits your criteria: it describes a person who is nice and pleasant to your face, but is doing terrible things to you in secret (like murdering your father).

The main drawback is that the phrase is not common enough to be called an "expression" or an "idiom". If the listener doesn't catch the literary reference, the phrase loses some of its impact, since the words of the phrase don't actually say who the villain is hurting. Fortunately, most English speakers have studied Hamlet in school at some point, so there is at least a chance that they will recognize it.

One last note: Shakespeare's definition of "villain" was not quite the definition we use today. In his usage, it describes someone who lacks noble qualities, in both senses of the word "noble": that which is good and admirable, and that which is characteristic of the nobility. (These concepts were more synonymous in Shakespeare's time than they are today.) See this question from the English Language & Usage StackExchange for further details. Anyone who hears you say the phrase will most likely assume the modern definition of "villain" ("a deliberate scoundrel or criminal"), which is close enough.

As you note, two-faced is a good expression for that. Backstabber has a similar meaning of "someone covertly taking actions against you without you knowing".

A wolf in sheep's clothing is similar as well, referring to someone who's outwardly a good person, but hides their true evil nature.

I would use backstabber:

One who attacks or betrays someone in a deceitful, underhanded, or treacherous way.
He proved to be a backstabber in business, assaulting the character of his competition to get ahead in the game.

I thought Mary was my friend, but then she said those mean things behind my back. She turned out to be nothing but a backstabber.

Though not quite an idiom, in the US, you can call such a person a Benedict Arnold.

Benedict Arnold was a military officer who served as a general for the American Continental Army during the American Revolutionary War. He famously betrayed the US and defected to the British. His name has since been used as a byword to mean someone who betrays their friends.

I suspect this wouldn't be used in BrE English that much.

play a double game;

be a double-dealer;

be double-faced;

be
two-faced;

carry fire in one hand and water in the other;

double
cross;

double-dealing;

double-faced tactics;

have two faces;

say
one thing and do another;
use dual tactics:

Double-tongued

Duplicity

double dealing and back stabbing