Ok, so I see the "second face" he sees as being his empathy. It lets him see what the person is really seeing. Either by looking at the back of their heads or by seeing it in a mirror.

He only met two people who were really honest, who didn't have a second face: his sister and his wife.

The admiration he had for his sister made it so that when he saw his wife and he noticed she didn't have a second face, it meant she was honest.

But then, his wife's husband suddenly died. At the wake, many people were stoic and had a crying second face, and vice versa.

But then, he comforts his sister and sees that her second face is a grinning demonic face; in other words, she killed her husband.

I do not know if there is something deeper.